ENG-4610: Make there one plan cache per process.
Merge remote-tracking branch 'origin/master' into ENG-4610-onecache
ENG-4610: Trivial feedback from Izzy.
Merge remote-tracking branch 'origin/master' into ENG-4610-onecache
Merge pull request #682 from VoltDB/ENG-4610-onecache

ENG-4610: Make there one plan cache per process.