lease: Lookup non-blocking with concurrent Grant/Revoke

[ximenzaoshi]

Contributing guidelines

lease: Change lease Mutex to RWMutex

When the lease gets huge, etcd costs too much time on lease map ranging, leading to updating requests timeout. Change the Mutex to RWMutex may be helpful.

related: #9159
[contributing]: https://github.com/coreos/etcd/blob/master/CONTRIBUTING.md#contribution-flow

[codecov-io]

Codecov Report

Merging #9229 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #9229      +/-   ##
==========================================
+ Coverage   75.87%   75.91%   +0.03%     
==========================================
  Files         363      363              
  Lines       30161    30161              
==========================================
+ Hits        22884    22896      +12     
+ Misses       5670     5663       -7     
+ Partials     1607     1602       -5

Impacted Files	Coverage Δ
lease/lessor.go	86.52% <100%> (ø)	⬆️
clientv3/namespace/watch.go	87.87% <0%> (-12.13%)	⬇️
etcdserver/api/v3rpc/lease.go	78.37% <0%> (-8.11%)	⬇️
etcdctl/ctlv3/command/lease_command.go	65.34% <0%> (-5.95%)	⬇️
pkg/testutil/recorder.go	77.77% <0%> (-3.71%)	⬇️
lease/leasehttp/http.go	62.5% <0%> (-2.95%)	⬇️
etcdserver/api/v3election/election.go	64.7% <0%> (-2.95%)	⬇️
etcdserver/api/v3rpc/watch.go	83.69% <0%> (-1.72%)	⬇️
etcdserver/cluster_util.go	76.64% <0%> (-1.46%)	⬇️
clientv3/op.go	72.56% <0%> (-1.33%)	⬇️
... and 16 more

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[ximenzaoshi]

cc @xiang90

[gyuho]

lgtm