Use hdel with many keys where supported

Deletion of multiple keys in one call was added in 2.4.0 so
we should try to use it if we are in that version (or newer) to
avoid doing a loop that does the same (optimizing bandwidth
and call count and such).

Change-Id: I145e75327fa4f826fa0e93d3270952089da9784f
Merge "Use hdel with many keys where supported"