Refactoring of 104067bc

Switch the MarkLive method for a chan that is closed by the client.
Thanks to @notnoop for the idea!

The old approach called a method on most existing ARs and TRs on every
runAllocs call. The new approach does a once.Do call in runAllocs to
accomplish the same thing with less work. Able to remove the gate
abstraction that did much more than was needed.