…grpc#8997)

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.
- Update the name generation algorithm to give higher preference to
reuse names of existing priority (if there is a locality match) over
lower priorities.
- 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:
* xds/priority: Stop caching child LB policies removed from the
configuration. This will help reduce memory and cpu usage when
localities are constantly switching between priorities.