introduce follower replication

[Fullstop000]

Signed-off-by: Fullstop000 fullstop1005@gmail.com

Implement follower replication

Relate to #11357

[codecov-io]

Codecov Report

Merging #11455 into master will increase coverage by 0.38%.
The diff coverage is 79.29%.

@@            Coverage Diff             @@
##           master   #11455      +/-   ##
==========================================
+ Coverage   66.08%   66.47%   +0.38%     
==========================================
  Files         401      403       +2     
  Lines       36627    36839     +212     
==========================================
+ Hits        24205    24488     +283     
+ Misses      10943    10837     -106     
- Partials     1479     1514      +35

Impacted Files	Coverage Δ
raft/raft.go	87.07% <71.07%> (-3.84%)	⬇️
raft/group.go	88.67% <88.67%> (ø)
clientv3/ordering/kv.go	16.17% <0%> (-69.12%)	⬇️
etcdserver/api/v3rpc/util.go	51.61% <0%> (-16.13%)	⬇️
clientv3/namespace/watch.go	87.87% <0%> (-6.07%)	⬇️
raft/tracker/inflights.go	91.83% <0%> (-4.09%)	⬇️
pkg/testutil/recorder.go	77.77% <0%> (-3.71%)	⬇️
etcdserver/api/v3compactor/revision.go	79.31% <0%> (-3.45%)	⬇️
pkg/adt/interval_tree.go	84.21% <0%> (-3.01%)	⬇️
proxy/grpcproxy/watch.go	89.94% <0%> (-2.96%)	⬇️
... and 19 more

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[hicqu]

@xiang90 Could we hold a meeting to discuss this? So we can speed up it.

[Fullstop000]

@xiang90 we've test this feature in TiKV and it results reducing some network transport cost. So we believe that this feature can facilitate cross-IDC transmission :)

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed after 21 days if no further activity occurs. Thank you for your contributions.