MEDIUM: connection: add an ->init function to data layer

SSL need to initialize the data layer before proceeding with data. At
the moment, this data layer is automatically initialized from itself,
which will not be possible once we extract connection from sessions
since we'll only create the data layer once the handshake is finished.

So let's have the application layer initialize the data layer before
using it.
diff --git a/include/types/connection.h b/include/types/connection.h
index 2fbd779..0e12bb5 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -143,6 +143,7 @@
 	void (*shutr)(struct connection *, int);    /* shutr function */
 	void (*shutw)(struct connection *, int);    /* shutw function */
 	void (*close)(struct connection *);         /* close the data channel on the connection */
+	int  (*init)(struct connection *conn);      /* initialize the data layer */
 };
 
 /* app_cb describes read and write callbacks which are called upon detected I/O