priority: implement changes to child policy caching specified in A115

[easwars]

Summary of changes:

• Add a new API to the balancergroup that allows removal of child policies with immediate effect, even when a cache is configured.
• Make the priority LB use this new API when removing child policies that have been removed from its configuration.
  • This change is protected with an environment variable that turns on the new behavior by default.
• Fix a bunch of tests in the priority LB policy to ensure that they work with the old and new behavior.
• Fix a couple of aggregate cluster tests such that they don't continuously handle the same NACKed resource from the management server.
• Other minor cleanups in tests that I had to look at as part of making this change.

RELEASE NOTES:

• TBD