Disable gRPC watch threads for 0.10.0
Summary: The refactor in https://dagster.phacility.com/D5953 is coming in too hot for 0.10.0, so this diff disables the watch threads until the webserver is properly refactored to support them.
Test Plan: unit
Reviewers: dgibson
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D5995