Add flag for bthread priority queue, the default is false

[chenBright]

What problem does this PR solve?

Issue Number: #3030

Problem Summary:

1. epoll bthread deal first #2819 has bugs(在高并发场景下server直接hang死 #3030).
2. Bugfix: Only event dispatcher thread has flag BTHREAD_GLOBAL_PRIORITY #3055 only fixes Socket IO，not all IO types.

What is changed and the side effects?

Changed:

Side effects:

• Performance effects:

• Breaking backward compatibility:

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[wwbmmm]

LGTM

[yanglimingcn]

LGTM

[lhsoft]

@chenBright 看起来是通过flag暂时关闭这个功能？如果我设置enable_bthread_priority_queue为true，还是会hung死吧

[chenBright]

@chenBright 看起来是通过flag暂时关闭这个功能？如果我设置enable_bthread_priority_queue为true，还是会hung死吧

现在只有epoll bthread才有BTHREAD_GLOBAL_PRIORITY，应该不会出现这个问题了吧。

[lhsoft]

@chenBright

现在只有epoll bthread才有BTHREAD_GLOBAL_PRIORITY，应该不会出现这个问题了吧。

FLAGS_event_dispatcher_num > 1 的情况下，应该还是会有多个吧

[chenBright]

@chenBright

现在只有epoll bthread才有BTHREAD_GLOBAL_PRIORITY，应该不会出现这个问题了吧。

FLAGS_event_dispatcher_num > 1 的情况下，应该还是会有多个吧

是的，后续找时间修一下这个问题。

[ZhengweiZhu]

_enable_priority_queue is not default initialized to false and may be true or false. I fixed it in #3096