CBQE-0: use strong reference to promise proxy

in latest version of celery promise methods cannot have weak references
- will fail to start worker

Change-Id: Ic34bb7c6b3bb4374d7e34dc6250a15eb9a0a6b75
Reviewed-on: http://review.couchbase.org/40350
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Tommie McAfee <tommie@couchbase.com>