WL#1211: Multiple query caches

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.