CBQE-0: Corrected place for expiration sleep in merge_buckets and moving some function from public to private

Change-Id: I59769664bc6fee2e61f7159d55a13ccb3904103e
Reviewed-on: http://review.couchbase.org/33906
Reviewed-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>
Tested-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>