asyncresult: Add convenience function to clear callbacks
asyncresult: Add convenience function to clear callbacks

This is typically quicker than doing a dict lookup and clearing both
fields individually.

Change-Id: I6471eaa5ea506343b50f67914652ae6876fed78c
Reviewed-on: http://review.couchbase.org/44964
Tested-by: Mark Nunberg <e7d42768707bf23038325b0b68d4b577e5f6064a@haskalah.org>
Reviewed-by: Volker Mische <fb414f8ac0dbbf87663550ae4ef5fc95b1041941@gmail.com>

mnunberg/couchbase-python-client,couchbase/couchbase-python-client,mnunberg/couchbase-python-client,couchbase/couchbase-python-client