Remove SchedulerActive, replace with explicit services.

This is the first part of a larger change that will allow us to
make pubsub events async as it makes the dependency on subscribers
processing a SchedulerActive event explicit.

Testing Done:
./gradlew -Pq build
test_end_to_end.sh

Bugs closed: AURORA-920

Reviewed at https://reviews.apache.org/r/27716/