feat(mito): Add writable flag to region

[evenyag]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

This PR adds a writable flag to each mito region and a set_writable() method to RegionEngine trait

• The engine opens regions in readonly mode
• To write a region, users need to call set_writable() to mark the region as writable

Note that the writable flag is a simple atomic flag, so it can't guarantee that the engine immediately cancels all running write operations.

Refactor

• It wraps Sender<Result<Output>> and Option<Sender<Result<Output>>> into new types OutputTx and OptionOutputtx.
• The region map provides two methods get_region_or() and writable_region_or(). They invoke the OnFailure callback if the region is unavailable.

The we can refactor the following code:

let region = match self.regions.get_writable_region(region_id) {
    Ok(v) => v,
    Err(e) => {
        request.on_failure(e);
        return;
    }
};

With writable_region_or()

let Some(region) = self.regions.writable_region_or(region_id, &mut request) else {
    return;
};

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

Merging #2349 (c892de7) into develop (93f3048) will decrease coverage by 0.02%.
The diff coverage is 81.42%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2349      +/-   ##
===========================================
- Coverage    77.49%   77.48%   -0.02%     
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  Files          716      716              
  Lines       113735   113827      +92     
===========================================
+ Hits         88141    88197      +56     
- Misses       25594    25630      +36     

[fengjiachun]

Is handle_alter also need to get writeable_region?

[killme2008]

LGTM

[fengjiachun]

LGTM