grpc-js: Ref and unref backoff timer

[murgatroid99]

We want the gRPC library to hold the Node process open if and only if there is an active request. The channel is responsible for this while a call has not yet been assigned to a subchannel, and the subchannel is responsible for this after the call has been assigned, until the call ends. In the subchannel, there is always either an http session, or an active backoff timer (or both). So, we ref both of those as long as the subchannel has at least one call. Since the resolving load balancer is never responsible for keeping the process open, we always unref its backoff timer. The same is true of the xDS client, which always operates as a subordinate of another channel.