External thread support

[bbeaupain]

We need an efficient (if possible) and safe way to queue I/O operations from external threads. Possibly just a simple synchronization mechanism and a ring wakeup call will be enough.

[siy]

The simplest approach might be to use a queue which supports multiple-producers-single-consumer (MPSC) access. Since performance and zero allocation are an absolute requirement, I'd suggest JCTools MpscArrayQueue.