xds/ringhash: cache connectivity state of subchannels inside picker

[dfawley]

I did not add a new test for this. The sequence of operations required here is fairly long and specific and requires races that are too difficult to reliably stimulate. Corresponding change in C++: grpc/grpc@98cdac3 (#32326). (gRPC-Java always kept a copy of the states in the picker.)

RELEASE NOTES:

• xds/ringhash: cache connectivity state of subchannels inside picker to avoid rare races