Add a `PriorityQueue`-for-idle-resources variant about reactor-pool HOT 1 OPEN

ConcurrentSkipListSet might do the trick.

As the last paper in the list mentioned, it's possible to implement a priority queue using concurrent skip lists, and as it happens Java has lock-free skip list implementation.

I had the freedom to write a simple concurrent priority queue using Java's ConcurrentSkipListSet and it works well with one drawback:

The given structure is set, which requires all elements to have distinct priorities. This was bypassed by adding an additional comparator that compares timestamps if user-defined priorities are equal.

Additional:
In addition to mentioned use cases, I as a user would like to define my own priorities.
Use case: if the resource is in charge of processing messages (eg. Mono<Result> processMessage(Message m) ), I would like to assign priority to messages that Reactor Pool would take into consideration when choosing who will get the resource.

Example:

Mono.just(getMessage())
.flatMap(msg -> pool.withPoolable(msg.priority(), resource -> resource.processMessage(msg));

from reactor-pool.