Prevent multiple duplicate callback executions.

If the Watcher.get() data is the same as the last time the
Watcher._execute_callbacks() method was called, theres no
reason to re-execute the callbacks ... so don't.
Reduce expected callback call_count in Watcher integration tests.
Pep8 cleanup
Merge branch 'master' of github.com:Nextdoor/ndserviceregistry into reduce_callback_execution
Merge pull request #45 from Nextdoor/reduce_callback_execution

Reduce callback execution