feat: introduce the peer state

[WenyXu]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

Introduces the peer state. By default, the peer's state is None.

We should downgrade a Peer before deactivating a Region on it:

• During the Region Failover Procedure (Region on the Peer is unavailable).
• Migrating a Region (Region on the Peer was planned to migrate to another Peer).

Notes: Meta Server will stop renewing the lease for a Region on the downgraded Peer.

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)

#2638

[codecov]

Codecov Report

Merging #2649 (a7e432a) into develop (01fdbf3) will decrease coverage by 0.36%.
Report is 1 commits behind head on develop.
The diff coverage is 94.28%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2649      +/-   ##
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- Coverage    84.90%   84.54%   -0.36%     
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  Files          743      743              
  Lines       120672   120704      +32     
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- Hits        102455   102050     -405     
- Misses       18217    18654     +437     

[MichaelScofield]

If some regions on a peer are under migration while others stay active, will this peer be marked as "downgraded"?

[WenyXu]

If some regions on a peer are under migration while others stay active, will this peer be marked as "downgraded"?

Nope, only region level downgrading.

[MichaelScofield]

If some regions on a peer are under migration while others stay active, will this peer be marked as "downgraded"?

Nope, only region level downgrading.

So you will set a peer's state to "downgraded" only when all the regions on it are under migration or deactivated?

[WenyXu]

So you will set a peer's state to "downgraded" only when all the regions on it are under migration or deactivated?

Oh, My bad.

If some regions on a peer are under migration while others stay active, will this peer be marked as "downgraded"?

The answer to this should be Yes. However, it might make the thing tricky... What if, most of the time, the crash happened at the Peer level? It might need to update lots of TableRoutes.