mvcc: no need to hold tx lock when converting kvs to watch events

[mlmhl]

Method kvsToEvents can take a long time if a large number of events need to be synchronized. However, this method does not access any shared resources, so it no need to hold the tx lock when executing. So we can move it outside to reduce lock holding time.

[codecov-io]

Codecov Report

Merging #11701 into master will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #11701      +/-   ##
==========================================
+ Coverage   66.36%   66.43%   +0.07%     
==========================================
  Files         402      402              
  Lines       36686    36686              
==========================================
+ Hits        24345    24371      +26     
+ Misses      10844    10811      -33     
- Partials     1497     1504       +7     

Impacted Files	Coverage Δ
mvcc/watchable_store.go	84.34% <100.00%> (-0.36%)	⬇️
auth/options.go	47.50% <0.00%> (-45.00%)	⬇️
auth/jwt.go	56.17% <0.00%> (-21.35%)	⬇️
etcdserver/api/v3rpc/util.go	51.61% <0.00%> (-16.13%)	⬇️
auth/simple_token.go	76.47% <0.00%> (-12.61%)	⬇️
etcdserver/api/v3rpc/lease.go	74.68% <0.00%> (-7.60%)	⬇️
clientv3/namespace/watch.go	87.87% <0.00%> (-6.07%)	⬇️
proxy/grpcproxy/lease.go	79.18% <0.00%> (-4.08%)	⬇️
etcdserver/util.go	95.23% <0.00%> (-3.58%)	⬇️
clientv3/leasing/cache.go	87.22% <0.00%> (-1.67%)	⬇️
... and 19 more

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[jingyih]

LGTM

[mlmhl]

@jingyih Could we merge this PR now?