rpc: tighter shutdown synchronization in client subscription (#22597)

This fixes a rare issue where the client subscription forwarding loop
would attempt send on the subscription's channel after Unsubscribe has
returned, leading to a panic if the subscription channel was already
closed by the user. Example:

    sub, _ := client.Subscribe(..., channel, ...)
    sub.Unsubscribe()
    close(channel)

The race occurred because Unsubscribe called quitWithServer to tell the
forwarding loop to stop sending on sub.channel, but did not wait for the
loop to actually come down. This is fixed by adding an additional channel
to track the shutdown, on which Unsubscribe now waits.

Fixes #22322