Gracefully switch balancers

[dfawley]

When switching balancers, we currently Close the old balancer, but keep using its Pick method. Also, we do not update the balancer when the state of its subconns change. We should keep using the old balancer more fully until the new balancer indicates it is ready.

[menghanl]

The first step is to stop using the picker after balancer is closed.
The fix for real graceful switch can happen later.

[stale]

This issue is labeled as requiring an update from the reporter, and no update has been received after 7 days. If no update is provided in the next 7 days, this issue will be automatically closed.

[dfawley]

Fixed by #5285