Switch socket management API to epoll() (#7785)

Switch socket management API to epoll() (#7785)

Previously all the server and client sockets were being managed by
the select() API, which operates using a fixed amount of client
sockets. This commit changes the code to adapt to the epoll() API.
This commit also could have a performance improvement due to epoll's
architecture.

Note: next commits should fix:
	- The clock from sec to millisecs as the epoll_wait()
	uses millisecs.
	- Unit tests need to be changed to work properly.