CBST-17: more accurate cache-miss ratio

the calculation for cache-miss was inaccurate, causing lower than expected misses.
when doing multi_get fix is to take a presentation of total gets and create a 'miss_batch' and then keep a hot_batch in memory to read with the misses.

Change-Id: Ia0b894c89fec1b33eca83fbb6f3ace8468f6c102
Reviewed-on: http://review.couchbase.org/30535
Reviewed-by: Tommie McAfee <tommie@couchbase.com>
Tested-by: Tommie McAfee <tommie@couchbase.com>