[FR] Add epoll update callback

[ethouris]

The updates done on SRT sockets can be, beside updates in the epoll system, highlighted also through a user function, in which a user would possibly also translate this signal into a signal for an event library of their choice.

This could give a user ability to - for example - implement this by setting a user event readiness, where the event is translated into a system event so that a system waiting function could be used instead of srt_epoll_wait. For example, on Linux users might use eventfd-created descriptors, in the update functions perform write into this descriptor, the value of the event flags, or 0 if the update effectively cleared off all event flags.

This would be an easy "epoll reimplementation" without the need of depending on an external library or platform-dependent solution.