adds a generic implementation of Gossip{Read,Write}Lock

[behzadnouri]

Problem

Need to improve lock contention in gossip.
But current hard-coded Gossip{Read,Write}Lock structs makes it difficult to
experiment with other sync constructs (e.g. parking_lot).

Summary of Changes

• added a generic implementation of Gossip{Read,Write}Lock.

[codecov]

Codecov Report

Merging #18559 (478eb34) into master (761de8b) will increase coverage by 0.0%.
The diff coverage is 94.1%.

@@           Coverage Diff           @@
##           master   #18559   +/-   ##
=======================================
  Coverage    82.7%    82.7%           
=======================================
  Files         442      442           
  Lines      123987   123975   -12     
=======================================
- Hits       102563   102560    -3     
+ Misses      21424    21415    -9