MEDIUM: connections: Provide a xprt_ctx for each xprt method.

For most of the xprt methods, provide a xprt_ctx.  This will be useful later
when we'll want to be able to stack xprts.
The init() method now has to create and provide the said xprt_ctx if needed.
diff --git a/src/connection.c b/src/connection.c
index 303f6df..2a66996 100644
--- a/src/connection.c
+++ b/src/connection.c
@@ -316,7 +316,7 @@
 	return ret;
 }
 
-int conn_unsubscribe(struct connection *conn, int event_type, void *param)
+int conn_unsubscribe(struct connection *conn, void *xprt_ctx, int event_type, void *param)
 {
 	struct wait_event *sw;
 
@@ -340,7 +340,7 @@
 	return 0;
 }
 
-int conn_subscribe(struct connection *conn, int event_type, void *param)
+int conn_subscribe(struct connection *conn, void *xprt_ctx, int event_type, void *param)
 {
 	struct wait_event *sw;