feat(mito): Stall write requests and add more flush tests

[evenyag]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

This PR implements the following feature for flush

• Rejecting write requests
  • reject_write_requests()
  • should_reject_write()
• Stalling write requests if WriteBufferManager::should_stall() returns true
  • StalledRequests holds stalled write requests
  • Once a flush job is finished, stalled requests can proceed. (In handle_flush_finished())

It also refactors tests and adds more test cases:

• It moves common functions to test_util.rs.
  • column_metadata_to_column_schema()
  • build_rows()
  • rows_schema()
  • delete_rows_schema()
  • put_rows()
  • build_rows_for_key()
  • build_delete_rows_for_key()
  • delete_rows()
• It moves flush tests to flush_test.rs.
• It adds an EventListener to listen to flush and stall events.
• Adds methods to set write buffer manager and event listener for the engine

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

• Tracking issue for implementing mito as a region engine #1869

[codecov]

Codecov Report

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Additional details and impacted files

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[v0y4g3r]

LGTM