Conflicts:
	CMakeLists.txt

fix #2576

Conflicts:
	src/network/socket_handler.hpp

Since struct sockaddr is too small to contain an IPv6, we use struct
sockaddr_in6 instead, and we cast it where needed

Use them to build without linking to them, even if they are on your system

SocketHandler has been renamed to TCPSocketHandler
SocketHandler is now a simple interface with a few methods, used only by
Poller.

This way we can inherite from the new SocketHandler class, to handle other
types of sockets, and still make them manageable by the poller without any
change in the Poller class.

fix #2551

fix #2552

See CMP0037

Was causing the event to not be removed in case of connection failure, which
lead to a dangling timed event named “timeout-1”, which was called later, by
some other SocketHandler for which even the socket creation failed (so, its
socket is -1), with a pointer to the previous SocketHandler which has
disappeared for a long time: segmentation fault etc.

ref #2455