lease: fix memory leak in LeaseGrant

[tangcong]

fix issue #11730 #11495

[tangcong]

leader can call leaseExpiredNotifier.Unregister() to pop item when lessor is revoked, but follower node has no way to pop it. it is not necessary to register it into the queue when lessor is not primary. if leader change, promote method can rebuild it in new leader and demote method can clear leaseExpiredNotifier in old leader.
other methods,it can also solve issues by removing item from leaseExpiredNotifier when executing lease revoking commands.

[codecov-io]

Codecov Report

Merging #11731 into master will decrease coverage by 0.38%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #11731      +/-   ##
==========================================
- Coverage   66.58%   66.19%   -0.39%     
==========================================
  Files         403      403              
  Lines       36763    36787      +24     
==========================================
- Hits        24477    24351     -126     
- Misses      10779    10942     +163     
+ Partials     1507     1494      -13     

Impacted Files	Coverage Δ
lease/lessor.go	88.00% <100.00%> (+0.08%)	⬆️
auth/store.go	52.61% <0.00%> (-22.78%)	⬇️
client/client.go	69.60% <0.00%> (-14.38%)	⬇️
proxy/grpcproxy/register.go	72.50% <0.00%> (-10.00%)	⬇️
clientv3/concurrency/election.go	79.68% <0.00%> (-2.35%)	⬇️
clientv3/maintenance.go	40.81% <0.00%> (-2.05%)	⬇️
etcdserver/api/v3rpc/watch.go	85.61% <0.00%> (-1.41%)	⬇️
etcdserver/cluster_util.go	64.18% <0.00%> (-0.94%)	⬇️
proxy/grpcproxy/lease.go	79.18% <0.00%> (-0.91%)	⬇️
clientv3/watch.go	90.60% <0.00%> (-0.84%)	⬇️
... and 15 more

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[gyuho]

great find. thanks for the fix

[ahrtr]

The fix is included in v3.4.6 (2020-03-29).