Most importantely, include parsing.h, since read_cfg is not implicitely
included anymore in botan 2.3, and that does not compile.
Also do not included botan.h anymore, since it’s deprecated in botan 2.3
fix#3296
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.