[#2598] Add Epoll.isAvailable() which allows to check if epoll can be used.

Motivation:

At the moment there is no simple way for a user to check if the native epoll transport can be used on the running platform. Thus the user can only try to instance it and catch any exception and fallback to nio transport.

Modification:

Add Epoll.isAvailable() which allows to check if epoll can be used.

Result:

User can easily check if epoll transport can be used or not