mvcc/backend: rename Lock() to RLock() in ReadTx interface

[jingyih]

For better code readability, renaming Lock() to RLock() in ReadTx interface.

This is my first attempt to address #10505. Personally I do not think we should wrap RLock() in a function named Lock(). But renaming the function does come with challenges. It is cause by the fact that ReadTx interface in embedded in BatchTx interface, and RLock() does not have a proper semantic under BatchTx interface.

Please let me know if this PR actually helps improve readability, or it adds more confusion.

FYI, I also added some comments to explain how the locks affect backend concurrency on reads and writes.

[jingyih]

/cc @xiang90 @gyuho @jpbetz

[codecov-io]

Codecov Report

Merging #10506 into master will increase coverage by 0.09%.
The diff coverage is 76.92%.

@@            Coverage Diff             @@
##           master   #10506      +/-   ##
==========================================
+ Coverage    71.4%   71.49%   +0.09%     
==========================================
  Files         393      393              
  Lines       36503    36511       +8     
==========================================
+ Hits        26064    26105      +41     
+ Misses       8604     8572      -32     
+ Partials     1835     1834       -1

Impacted Files	Coverage Δ
mvcc/kvstore_txn.go	65.95% <100%> (ø)	⬆️
mvcc/kvstore.go	85.35% <100%> (ø)	⬆️
mvcc/watchable_store.go	83.5% <100%> (-0.71%)	⬇️
mvcc/backend/read_tx.go	84.31% <100%> (+0.64%)	⬆️
mvcc/backend/backend.go	74.15% <100%> (ø)	⬆️
mvcc/backend/batch_tx.go	57.3% <57.14%> (-0.84%)	⬇️
clientv3/concurrency/election.go	79.68% <0%> (-2.35%)	⬇️
etcdctl/ctlv3/command/printer_simple.go	72.48% <0%> (-1.35%)	⬇️
pkg/adt/interval_tree.go	82.28% <0%> (-1.21%)	⬇️
etcdserver/api/rafthttp/peer.go	78.77% <0%> (-1.12%)	⬇️
... and 18 more

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[xiang90]

lgtm