CBD-926: Move bucket_engine to this repo

bucket_engine is tightly coupled with memcached and in such
a stable state that we don't have much benefit of keeping
it in its separate repository (with the drawbacks of
running autoconf etc to build it)

Change-Id: If5d570ed81eb3e5e6d1adb7bc66c7c007c8d8b1e
Reviewed-on: http://review.couchbase.org/25367
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>