Optimize libvbucket populate_bucket.

libvbucket called cJSON_GetArrayItem recursively,
which itself is a loop over many objects.  Removed
the need for this triple embedded loop.

Change-Id: I338467cdfe17b139b8339921063dd2f13c804e8d
Reviewed-on: http://review.couchbase.org/34917
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Subhashni Balakrishnan <b.subhashni@gmail.com>
Tested-by: Brett Lawson <brett19@gmail.com>