Motivation
==========
Provide users with ability to know if result is active or replica.

Modification
============
Add is_active flag to GetReplicaResult.  Update tests to verify.

Results
=======
All tests pass.

Change-Id: Ifa52c15d7d5c7fc25aa2d64b1a1bd8bb8d416f68
Reviewed-on: http://review.couchbase.org/c/couchbase-python-client/+/164378
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: David Kelly <davidmichaelkelly@gmail.com>