MB-100: replaced orddict:find with lists:keysearch

Which we already know is much faster. Overall effect on bucket stats
building appears to be shaving off about 1/3rd of time.

Change-Id: I6393cccf204eb8c16551bdda68ed86993f3b0651
Reviewed-on: http://review.couchbase.org/26362
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>