raft: reduce heartbeat msg

[NingLin-P]

Signed-off-by: linning [email protected]

When there is no pending Readindex, MsgAppend will do the work of MsgHeartbeat, so when we had broadcasted MsgAppend, we can reset heartbeat_elapsed to reduce MsgHeartbeat.

also see: tikv/raft-rs#309 tikv/raft-rs#313

[codecov-io]

Codecov Report

Merging #11347 into master will decrease coverage by 0.13%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #11347      +/-   ##
==========================================
- Coverage    64.6%   64.47%   -0.14%     
==========================================
  Files         403      403              
  Lines       37969    37973       +4     
==========================================
- Hits        24531    24483      -48     
- Misses      11805    11847      +42     
- Partials     1633     1643      +10

Impacted Files	Coverage Δ
raft/raft.go	89.96% <100%> (-0.59%)	⬇️
proxy/httpproxy/director.go	61.42% <0%> (-18.58%)	⬇️
proxy/grpcproxy/register.go	69.44% <0%> (-13.89%)	⬇️
clientv3/naming/grpc.go	63.15% <0%> (-12.29%)	⬇️
client/members.go	56.45% <0%> (-8.88%)	⬇️
clientv3/leasing/util.go	91.66% <0%> (-6.67%)	⬇️
client/client.go	78.43% <0%> (-5.56%)	⬇️
clientv3/leasing/cache.go	87.22% <0%> (-4.45%)	⬇️
proxy/grpcproxy/watcher.go	89.79% <0%> (-4.09%)	⬇️
clientv3/leasing/txn.go	88.09% <0%> (-3.18%)	⬇️
... and 13 more

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