MINOR: connections: Add a new mux method, "takeover".

Add a new mux method, "takeover", that will attempt to make the current thread
responsible for the connection.
It should return 0 on success, and non-zero on failure.
diff --git a/include/types/connection.h b/include/types/connection.h
index 30cb895..0136be9 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -369,6 +369,7 @@
 	void (*reset)(struct connection *conn); /* Reset the mux, because we're re-trying to connect */
 	const struct cs_info *(*get_cs_info)(struct conn_stream *cs); /* Return info on the specified conn_stream or NULL if not defined */
 	int (*ctl)(struct connection *conn, enum mux_ctl_type mux_ctl, void *arg); /* Provides informations about the mux */
+	int (*takeover)(struct connection *conn); /* Attempts to migrate the connection to the current thread */
 	unsigned int flags;                           /* some flags characterizing the mux's capabilities (MX_FL_*) */
 	char name[8];                                 /* mux layer name, zero-terminated */
 };