synchronize while-committing callbacks in CommitBarrier

[nbronson]

While-committing callbacks happen after the commit decision but before the effects of the transaction have become visible. We should extend the CommitBarrier semantics so that no effects from any commit barrier member become visible until all of the while-committing callbacks have completed.

One way to do this would be to add another callback method to the ExternalDecider trait, perhaps endCommitting. The commit barrier implementation would then perform a second barrier at this point. Various optimizations are possible if the second barrier only has arrivals for members that are known to have while-committing callbacks.