* Do not post to closed channels.
Perils of atomics. Hard to imagine, but I guess it could happen.
The postMaxLayerNotifier checked for closed and down track was not
closed. But, between that check and posting to channel (which is
a very small window), the down track could have been closed and
the channel (maxLayerNotiferCh) is closed.
Protect that channel post + close with the bind lock.
* reduce the change
* Check for closed inside lock