CLEANUP: connection: rename conn->mux_ctx to conn->ctx

We most often store the mux context there but it can also be something
else while setting up the connection. Better call it "ctx" and know
that it's the owner's context than misleadingly call it mux_ctx and
get caught doing suspicious tricks.
diff --git a/include/types/connection.h b/include/types/connection.h
index 430e92a..f1fd7a0 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -429,7 +429,7 @@
 	const struct xprt_ops *xprt;  /* operations at the transport layer */
 	const struct mux_ops  *mux;   /* mux layer opreations. Must be set before xprt->init() */
 	void *xprt_ctx;               /* general purpose pointer, initialized to NULL */
-	void *mux_ctx;                /* mux-specific context, initialized to NULL */
+	void *ctx;                    /* highest level context (usually the mux), initialized to NULL */
 	void *owner;                  /* pointer to the owner session, or NULL */
 	enum obj_type *target;        /* the target to connect to (server, proxy, applet, ...) */