The pick_first policy creates a list of subchannels for each resolver update and then iterates over the list, attempting to connect to each subchannel in turn, until one of them succeeds. However, once a subchannel does succeed, the policy unrefs the other subchannels but still retains a bunch of now-unnecessary state in the subchannel list itself. This wastes a bunch of memory, especially now that petiole policies are delegating to pick_first. This PR contains a new pick_first implementation that stops retaining that state, which significantly reduces per-channel memory.
There is one behavior change here, which is that if we have a connected subchannel and we get a resolver update that no longer includes that address, we now go IDLE instead of proactively trying to connect to the new addresses.
Closes#34766
COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/34766 from markdroth:pick_first_free_memory_after_connecting 7236b4321f
PiperOrigin-RevId: 623887639
696d644bcf