CBIDXT-53 Initial Implementation of SlabManager

Slab Manager provides a wrapper around go-slab library to allow concurrent access and
memory accounting(e.g. limit the max memory of Arena etc).

Change-Id: I15a637e9876e94d757272f39fe7ec4434390f381
Reviewed-on: http://review.couchbase.org/36295
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>