Add metadata cache (as rebased by jtuple)

This change adds an ETS table per vnode to act as a cache for the vclocks and index specs of any values that have recently been gotten or put. This is meant to save round-trips to the disk in common get-then-put workflows on slower disks.

The cache is size limited, and setting the size to 0 will disable it, since it may be slight performance hit in certain situations (e.g. using the memory backend, when all of your data fits into RAM, etc).