MB-20054: Account for memory alloc/dealloc in unregisterBucket

MB-20054: Account for memory alloc/dealloc in unregisterBucket

While unregistering a bucket, any memory allocations/deallocations
made should be accounted to the bucket in question.  Hence no
`onSwitchThread(NULL)` call.

Change-Id: I5c260e3aa7e2c8d1fd4ff0a1ca20f2185a7362a8
Reviewed-on: http://review.couchbase.org/65525
Well-Formed: buildbot <build@couchbase.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Will Gardner <will.gardner@couchbase.com>
Reviewed-by: Jim Walker <jim@couchbase.com>