grpc-js: Fix pick_first reconnecting without active calls

[murgatroid99]

This is another attempt at fixing #2718. The previous attempt, #2680, failed because of #2690. I believe I have fixed that this time. With the original change, I reproduced a failure like the one reported: if the server dropped each connection after 1 second, and the client made requests as quickly as possible, after only a couple of minutes the client would entirely stop attempting to reconnect. With the change to handle ready subchannels in connectToAddressList, I ran the same test for half an hour without any problem.