Add a level of inheritance above SocketHandler

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.

src/irc/irc_client.cpp

@@ -20,7 +20,7 @@ using namespace std::string_literals;
 using namespace std::chrono_literals;
 
 IrcClient::IrcClient(std::shared_ptr<Poller> poller, const std::string& hostname, const std::string& username, Bridge* bridge):
-  SocketHandler(poller),
+  TCPSocketHandler(poller),
   hostname(hostname),
   username(username),
   current_nick(username),