rafthttp: make "ActiveSince" non-blocking on write lock

[gyuho]

"ActiveSince" is read-only, and should be able to call
concurrently, as long as there is no routine holding
write lock (send, send snapshot).

e.g. "ActiveSince" is used around etcd server
"processInternalRaftRequestOnce", should be non-blocking

[codecov-io]

Codecov Report

Merging #9366 into master will decrease coverage by 0.24%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #9366      +/-   ##
=========================================
- Coverage   72.74%   72.5%   -0.25%     
=========================================
  Files         361     361              
  Lines       30717   30717              
=========================================
- Hits        22346   22272      -74     
- Misses       6766    6828      +62     
- Partials     1605    1617      +12

Impacted Files	Coverage Δ
rafthttp/transport.go	83.24% <100%> (ø)	⬆️
etcdctl/ctlv3/command/printer.go	22.64% <0%> (-21.7%)	⬇️
pkg/tlsutil/tlsutil.go	86.2% <0%> (-6.9%)	⬇️
clientv3/namespace/watch.go	72.72% <0%> (-6.07%)	⬇️
etcdserver/api/v3rpc/lease.go	77.02% <0%> (-5.41%)	⬇️
proxy/grpcproxy/watcher.go	89.79% <0%> (-4.09%)	⬇️
etcdserver/v2_server.go	80.76% <0%> (-3.85%)	⬇️
etcdserver/api/v3election/election.go	64.7% <0%> (-2.95%)	⬇️
etcdserver/api/v3rpc/watch.go	81.27% <0%> (-2.56%)	⬇️
pkg/netutil/netutil.go	66.66% <0%> (-2.09%)	⬇️
... and 116 more

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[xiang90]

Is this change purely for performance? Or there might be a deadlock somewhere if trying to hold a write lock?

[gyuho]

@xiang90 Just minor performance improvements.

Found out while adding another method to rafthttp #9364.

[xiang90]

lgtm