+act - #15501 - Adding support for a Non-blocking, bounded, MPMC mailbox...

... called 'NonBlockingBoundedMailbox'

See #15501 
If thread A calls getDeq().count then gets suspended, and then the poller thread consumes X entries and inserter threads insert Y entries, by the time thread A is resumed it will return just garbage. In the current version it will refrain from producing garbage. That's the bigger difference. But I'm open to the original version with `min` if we are inclined to do so!