WL#1211: Multiple query caches

Affects: Server-7.1   —   Status: Un-Assigned   —   Priority: Medium

Allow user to have several query caches with query hashes evenly distributed
among them, to remove possibly hotspot  and especially to have maintainance
commands not blocking anything for a long queries.   

FLUSH QUERY CACHE even for 1G query cache may take a while, while other commands
we might wish to have, such as SHOW QUERY CACHE CONTENT might lock it for even
longer time.