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When the connection is lost, immediately try to reconnect, then try to
reconnect every 2 seconds.  This is much better than the previous “Try to
re-connect as fast as possible”.

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.

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.

This actually makes the session_manager be useful, and saves a few octets of
memory for the other ones

For now, it tries two TLS ports and then connects to the non-tls port.  In
the future we would like the user to be able to configure that.

fix #2435

Since the socket is now only created and managed whenever the connection is
being established, we only close the socket and if it was created (we use -1
to denote the fact that is not yet created, or has been closed) and we only
unmanage the socket if it is effectively managed.

fix #2529

fix #2522

We want to call socket() with the parameters provided by getaddrinfo, so we
can’t addd the fd into the poller immediately. We need to wait the
connection attempt, and then the SocketHandler can call add_socket_handler
itself, if the connection succeeds, or is in progress.

Apparently on some systems, subsquent connect() calls may fail with EISCONN
error, to indicate that the connection succeded in the background, instead
of returning 0.

The error is handled using the return value

(that is, the read/write/connect/etc)
Because this code is actually common for both the IrcClient and
XmppComponent class. These two classes have to implement some higher level
callbacks (parsing the data provided in the buffers, doing stuff when the
connection is done) instead of doing the read/write/connect low level
things.