CBST-15: fix for kv_latency stat collection

the new workloader creates a document key_map.
that is instead of putting all keys into ccq
it simply puts the start-end key of a range
whereby the keys can be reconstructed.  therefore
when reading from ccq, we can use just the start key
for checking kv latency

Change-Id: I624e9962d2dc1880666438fb689c716e836916fd
Reviewed-on: http://review.couchbase.org/30533
Reviewed-by: Tommie McAfee <tommie@couchbase.com>
Tested-by: Tommie McAfee <tommie@couchbase.com>