PYCBC-1171: Cleanup txcoucbhase API AsynResult callback references
PYCBC-1171: Cleanup txcoucbhase API AsynResult callback references

Motivation
==========
The txcouchbase API would set callbacks for AsyncResult objects
without also clearing the callbacks once the callback (or errback) is
called.  The behavior meant that the callback and errback had a dangling
reference and therefore unable to be deallocated.  From the user perspective unbounded memory increases were possible.

Modification
============
Update how the txcouchbase API handles setting and clearing callbacks
for the AsyncResult.

Results
=======
Memory does not increase unbounded and all tests pass.

Change-Id: I0941c494fa0c0b499ddb6d1b24058ac613aa88b1
Reviewed-on: http://review.couchbase.org/c/couchbase-python-client/+/160748
Reviewed-by: David Kelly <d83cd2a93a916ce0b0d4c1349b44be044799feca@gmail.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

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