Throw an exception has two dissadvantages on very high load
application: (1) it flushes CPU caches, and (2) it allocates memory to
build a stack trace.

Here I've introduced a way to save tons of CPU time by allow user to
return excepted result (`null` for example) when queue is empty.

Right now one queue is implemented it, I just added it as official
interface to `PriorityQueue`.

This is a copy of `ArrayFIFOQueue` which has fixed capacity.