JCBC-254: handle NOT_FOUND responses in observe() method

This is primarily for delete observe situations, where the item
is expecting to transition to NOT_FOUND.

Change-Id: If1c7d3aff0940a6534dbabf138eb95ba2df701f1
Reviewed-on: http://review.couchbase.org/24886
Tested-by: Matt Ingenthron <matt@couchbase.com>
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>