push data asynchronously at leader

[JacksonYao287]

now we always start appending log to log store at leader until all the futures of pushing data to followers are completed , which will involve unnecessary latency

this PR aims to make pushing data asynchronously, so that logs can be set to follower ASAP. it does not matter whether push_data is successful, since follower will try to fetch data from the leader if push_data fails

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Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 67.17%. Comparing base (1a0cef8) to head (dd3156d).
Report is 91 commits behind head on master.

Files with missing lines	Patch %	Lines
src/lib/replication/repl_dev/raft_repl_dev.cpp	66.66%	1 Missing ⚠️

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@@             Coverage Diff             @@
##           master     #590       +/-   ##
===========================================
+ Coverage   56.51%   67.17%   +10.65%     
===========================================
  Files         108      109        +1     
  Lines       10300    10729      +429     
  Branches     1402     1466       +64     
===========================================
+ Hits         5821     7207     +1386     
+ Misses       3894     2824     -1070     
- Partials      585      698      +113     

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[xiaoxichen]

it seems right but I would like to hold this till we solve and bake a stable version for PG move.

[JacksonYao287]

it seems right but I would like to hold this till we solve and bake a stable version for PG move.

sure, will also loop other members in

[xiaoxichen]

one question:
the push_data_to_all_followers returns a future, we dont wait for the future at all, what is the difference compared to run_on_forget

[JacksonYao287]

we dont wait for the future at all

we use folly::collectAllUnsafe to wait for the completion of all the future, no?

[xiaoxichen]

collectAllUnsafe

https://github.com/facebook/folly/blob/main/folly/futures/Future.h#L2360-L2385
it returns a future which satisfied when all to_be_collected features satisfied.

[JacksonYao287]

collectAllUnsafe

I see , you are right. I misunderstood the meaning of collectAllUnsafe. now I think , as you said , there is no difference compared to run_on_forget and the only improvement in this PR is that allocating_buffer and releasing buffer will not block the current thread, which seems not worth a change. let me close this pr