app: fix crash on logrotate and immediate exit

`say_logrotate` does not rotate logs synchronously. It posts tasks to
coio which executes them in it's pool thread. On application exit we
destroy logs calling `log_destroy`. This function waits for rotate task
to finish because if it will free resources immediately then unfinished
rotate task can have use-after-free issues. Waiting crashes because at
this moment event loop is not running which is required for
`fiber_cond_wait` to work.

Note that if there is no crash then we will hang forever waiting in
`log_destroy` because event loop is not running and
`log_rotate_async_cb` will never be executed.

Let's use mutexes and conditions instead. It solves both problems with
crash and hang. Thanks Vladimir Davydov for idea.

Fixes #4450

NO_DOC=bugfix