Cache child policies after deletion only where appropriate

[easwars]

Currently the balancergroup package is used by a bunch of LB policies:

• priority
• rls
• weightedtarget
• clustermanager

While the balancergroup has functionality to cache removed LB policies for a pre-defined amount of time, this functionality was supposed to be only used by the priority LB policy. But this functionality is currently used by the other policies listed above as well.

We have received reports of an issue where the number of open connections held by the gRPC client is seeing ephemeral increases. This could well be because of the caching of removed LB policies and their subConns, especially given that the clustermanager is the top-level LB policy.

We should make this behavior in the balancergroup configurable with the default behavior being no caching.