Add io finished callback to RaftLogStorage::save_vote

[ukernel]

Checklist

• Updated guide with pertinent info (may not always apply).
• Squash down commits to one or two logical commits which clearly describe the work you've done.
• Unittest is a friend:)

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This change is 

[drmingdrmer]

It's a lot changes :)

Can you explain the design purpose of this PR?

And since LogEventChannel is the major component of this PR, it requires some doc comment explaining the function of it.
And test should be added to such an important component.

And I do not quite get it why do you use the notify channel for receiving callback event. It's designed to receive all of the internal event.
https://github.com/datafuselabs/openraft/blob/30cdf5fbc9275ce6213ca8f70cf37bc0c6fe957a/openraft/src/core/raft_core.rs#L201

Reviewable status: 0 of 11 files reviewed, all discussions resolved