feat(mito): Checks whether a region should flush periodically

[evenyag]

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What's changed and what's your intention?

This PR checks whether a region should flush periodically and flush it if its last flush time is earlier than auto_flush_interval. The previous implementation only triggers the check on flush, which implies the write buffer is full. It might cause the database to hold a large amount of data for a long time if users stop ingesting data.

This PR

• adds a timeout to receive a channel so the worker can handle some tasks periodically
• adds a TimeProvider to get the current timestamp and checks elapsed time, this also simplifies test implementation

Each worker will start with a random check delay between [0, MAX_INITIAL_CHECK_DELAY_SECS) to avoid them flush together.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

[codecov]

Codecov Report

Attention: Patch coverage is 97.05882% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 85.00%. Comparing base (b85d7bb) to head (a4145ca).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3459      +/-   ##
==========================================
- Coverage   85.30%   85.00%   -0.31%     
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  Files         904      906       +2     
  Lines      150170   150925     +755     
==========================================
+ Hits       128106   128288     +182     
- Misses      22064    22637     +573     

[fengjiachun]

LGTM