WL#855: Priority Caching (Keycache)

Implemnt feature which will allow some tables/indexes to get better priority for
caching, to serve real-time like quiries.

There are two possible ideas:

1) Have some sort of index priority which is to be used in key buffer block
removal policy. 

2) Implement several keycaches and allow mapping tables to them.
This requires more changes and needs additional syntax for setting index->buffer
maps.