Rewrite queue (#24505)

# ⚠️ Breaking

Many deprecated queue config options are removed (actually, they should
have been removed in 1.18/1.19).

If you see the fatal message when starting Gitea: "Please update your
app.ini to remove deprecated config options", please follow the error
messages to remove these options from your app.ini.

Example:

```
2023/05/06 19:39:22 [E] Removed queue option: `[indexer].ISSUE_INDEXER_QUEUE_TYPE`. Use new options in `[queue.issue_indexer]`
2023/05/06 19:39:22 [E] Removed queue option: `[indexer].UPDATE_BUFFER_LEN`. Use new options in `[queue.issue_indexer]`
2023/05/06 19:39:22 [F] Please update your app.ini to remove deprecated config options
```

Many options in `[queue]` are are dropped, including:
`WRAP_IF_NECESSARY`, `MAX_ATTEMPTS`, `TIMEOUT`, `WORKERS`,
`BLOCK_TIMEOUT`, `BOOST_TIMEOUT`, `BOOST_WORKERS`, they can be removed
from app.ini.

# The problem

The old queue package has some legacy problems:

* complexity: I doubt few people could tell how it works.
* maintainability: Too many channels and mutex/cond are mixed together,
too many different structs/interfaces depends each other.
* stability: due to the complexity & maintainability, sometimes there
are strange bugs and difficult to debug, and some code doesn't have test
(indeed some code is difficult to test because a lot of things are mixed
together).
* general applicability: although it is called "queue", its behavior is
not a well-known queue.
* scalability: it doesn't seem easy to make it work with a cluster
without breaking its behaviors.

It came from some very old code to "avoid breaking", however, its
technical debt is too heavy now. It's a good time to introduce a better
"queue" package.

# The new queue package

It keeps using old config and concept as much as possible.

* It only contains two major kinds of concepts:
    * The "base queue": channel, levelqueue, redis
* They have the same abstraction, the same interface, and they are
tested by the same testing code.
* The "WokerPoolQueue", it uses the "base queue" to provide "worker
pool" function, calls the "handler" to process the data in the base
queue.
* The new code doesn't do "PushBack"
* Think about a queue with many workers, the "PushBack" can't guarantee
the order for re-queued unhandled items, so in new code it just does
"normal push"
* The new code doesn't do "pause/resume"
* The "pause/resume" was designed to handle some handler's failure: eg:
document indexer (elasticsearch) is down
* If a queue is paused for long time, either the producers blocks or the
new items are dropped.
* The new code doesn't do such "pause/resume" trick, it's not a common
queue's behavior and it doesn't help much.
* If there are unhandled items, the "push" function just blocks for a
few seconds and then re-queue them and retry.
* The new code doesn't do "worker booster"
* Gitea's queue's handlers are light functions, the cost is only the
go-routine, so it doesn't make sense to "boost" them.
* The new code only use "max worker number" to limit the concurrent
workers.
* The new "Push" never blocks forever
* Instead of creating more and more blocking goroutines, return an error
is more friendly to the server and to the end user.

There are more details in code comments: eg: the "Flush" problem, the
strange "code.index" hanging problem, the "immediate" queue problem.

Almost ready for review.

TODO:

* [x] add some necessary comments during review
* [x] add some more tests if necessary
* [x] update documents and config options
* [x] test max worker / active worker
* [x] re-run the CI tasks to see whether any test is flaky
* [x] improve the `handleOldLengthConfiguration` to provide more
friendly messages
* [x] fine tune default config values (eg: length?)

## Code coverage:

![image](https://user-images.githubusercontent.com/2114189/236620635-55576955-f95d-4810-b12f-879026a3afdf.png)

docs/content/doc/administration/config-cheat-sheet.zh-cn.md

@@ -43,7 +43,6 @@ menu:
 - `DEFAULT_PRIVATE`: 默认创建的git工程为私有。 可以是`last`, `private` 或 `public`。默认值是 `last`表示用户最后创建的Repo的选择。
 - `DEFAULT_PUSH_CREATE_PRIVATE`: **true**:  通过 ``push-to-create`` 方式创建的仓库是否默认为私有仓库.
 - `MAX_CREATION_LIMIT`: 全局最大每个用户创建的git工程数目， `-1` 表示没限制。
-- `PULL_REQUEST_QUEUE_LENGTH`: 小心：合并请求测试队列的长度，尽量放大。
 
 ### Repository - Release (`repository.release`)
 
@@ -111,10 +110,6 @@ menu:
 - `ISSUE_INDEXER_CONN_STR`: ****: 工单索引连接字符串，仅当 ISSUE_INDEXER_TYPE 为 `elasticsearch` 时有效。例如: http://elastic:changeme@localhost:9200
 - `ISSUE_INDEXER_NAME`: **gitea_issues**: 工单索引名称，仅当 ISSUE_INDEXER_TYPE 为 `elasticsearch` 时有效。
 - `ISSUE_INDEXER_PATH`: **indexers/issues.bleve**: 工单索引文件存放路径，当索引类型为 `bleve` 时有效。
-- `ISSUE_INDEXER_QUEUE_TYPE`: **levelqueue**: 工单索引队列类型，当前支持 `channel`， `levelqueue` 或 `redis`。
-- `ISSUE_INDEXER_QUEUE_DIR`: **indexers/issues.queue**: 当 `ISSUE_INDEXER_QUEUE_TYPE` 为 `levelqueue` 时，保存索引队列的磁盘路径。
-- `ISSUE_INDEXER_QUEUE_CONN_STR`: **addrs=127.0.0.1:6379 db=0**: 当 `ISSUE_INDEXER_QUEUE_TYPE` 为 `redis` 时，保存Redis队列的连接字符串。
-- `ISSUE_INDEXER_QUEUE_BATCH_NUMBER`: **20**: 队列处理中批量提交数量。
 
 - `REPO_INDEXER_ENABLED`: **false**: 是否启用代码搜索（启用后会占用比较大的磁盘空间，如果是bleve可能需要占用约6倍存储空间）。
 - `REPO_INDEXER_TYPE`: **bleve**: 代码搜索引擎类型，可以为 `bleve` 或者 `elasticsearch`。
@@ -122,7 +117,6 @@ menu:
 - `REPO_INDEXER_CONN_STR`: ****: 代码搜索引擎连接字符串，当 `REPO_INDEXER_TYPE` 为 `elasticsearch` 时有效。例如： http://elastic:changeme@localhost:9200
 - `REPO_INDEXER_NAME`: **gitea_codes**: 代码搜索引擎的名字，当 `REPO_INDEXER_TYPE` 为 `elasticsearch` 时有效。
 
-- `UPDATE_BUFFER_LEN`: **20**: 代码索引请求的缓冲区长度。
 - `MAX_FILE_SIZE`: **1048576**: 进行解析的源代码文件的最大长度，小于该值时才会索引。
 
 ## Security (`security`)