Raft HTTP: fix pause/resume race condition

[yznima]

Please read https://github.com/etcd-io/etcd/blob/master/CONTRIBUTING.md#contribution-flow.

[xiang90]

lgtm. i am wondering how do you find this issue?

[yznima]

lgtm. i am wondering how do you find this issue?

Running the test with the --race flag

==================
WARNING: DATA RACE
Read at 0x00c00055e5d8 by goroutine 110:
github.com/appian/etcd/rafthttp.(*Transport).Pause()
....

Previous write at 0x00c00055e5d8 by main goroutine:
github.com/appian/etcd/rafthttp.(*Transport).Start()
....
==================

[codecov-io]

Codecov Report

Merging #10826 into master will increase coverage by 0.75%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #10826      +/-   ##
==========================================
+ Coverage   62.79%   63.54%   +0.75%     
==========================================
  Files         391      391              
  Lines       37318    37322       +4     
==========================================
+ Hits        23433    23716     +283     
+ Misses      12297    12016     -281     
- Partials     1588     1590       +2

Impacted Files	Coverage Δ
etcdserver/api/rafthttp/transport.go	84.16% <100%> (+0.29%)	⬆️
client/members.go	65.32% <0%> (-20.17%)	⬇️
client/client.go	78.43% <0%> (-5.56%)	⬇️
pkg/transport/listener.go	51.18% <0%> (-3.8%)	⬇️
etcdserver/api/v3rpc/watch.go	77.77% <0%> (-2.62%)	⬇️
etcdserver/v3_server.go	70.61% <0%> (-1.37%)	⬇️
etcdserver/api/v2http/client.go	84.3% <0%> (-1.21%)	⬇️
clientv3/client.go	76.83% <0%> (-1.13%)	⬇️
clientv3/balancer/grpc1.7-health.go	59.01% <0%> (-0.88%)	⬇️
raft/raft.go	91.51% <0%> (-0.7%)	⬇️
... and 22 more

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