Reduce excessive memory usage by stats_prefix.
Reduce excessive memory usage by stats_prefix.

A key without prefix is also registered in stats_prefix structure.
It uses too much memory and lowers the performance in finding prefixes.
So, I made the key not having prefix is not registered in stats_prefix.

steveyen/moxi,membase/moxi,membase/moxi,zbase/moxi,zbase/moxi,couchbase/moxi,couchbase/moxi,membase/moxi,couchbase/moxi,steveyen/moxi,zbase/moxi,couchbase/moxi,couchbase/moxi,steveyen/moxi,membase/moxi,steveyen/moxi,steveyen/moxi,membase/moxi,zbase/moxi,membase/moxi,zbase/moxi,couchbase/moxi