MINOR: connections: Add a new xprt method, remove_xprt.

Add a new method to xprt_ops, remove_xprt. When called, if the provided
xprt_ctx is the same as the xprt's underlying xprt_ctx, it then uses the
new xprt provided, otherwise it calls the remove_xprt method of the next
xprt.
The goal is to be able to add a temporary xprt, that removes itself from
the chain when it did what it had to do. This will be used to implement
a pseudo-xprt for anything that just requires a handshake (such as the
proxy protocol).
diff --git a/include/types/connection.h b/include/types/connection.h
index e6b3ed5..e513994 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -333,6 +333,7 @@
 	char name[8];                               /* transport layer name, zero-terminated */
 	int (*subscribe)(struct connection *conn, void *xprt_ctx, int event_type, void *param); /* Subscribe to events, such as "being able to send" */
 	int (*unsubscribe)(struct connection *conn, void *xprt_ctx, int event_type, void *param); /* Unsubscribe to events */
+	int (*remove_xprt)(struct connection *conn, void *xprt_ctx, void *toremove_ctx, const struct xprt_ops *newops, void *newctx); /* Remove an xprt from the connection, used by temporary xprt such as the handshake one */
 };
 
 /* mux_ops describes the mux operations, which are to be performed at the