MINOR: checks: use cs_drain_and_close() instead of draining the connection

As explained in previous commit, the situation is absurd as we try to
cleanly drain pending data before impolitely shutting down, and it could
be counter productive on real muxes. Let's use cs_drain_and_close() instead.
diff --git a/src/check.c b/src/check.c
index edcaf32..c625b90 100644
--- a/src/check.c
+++ b/src/check.c
@@ -797,8 +797,7 @@
 		/* Check complete or aborted. If connection not yet closed do it
 		 * now and wake the check task up to be sure the result is
 		 * handled ASAP. */
-		conn_sock_drain(conn);
-		cs_close(cs);
+		cs_drain_and_close(cs);
 		ret = -1;
 		/* We may have been scheduled to run, and the
 		 * I/O handler expects to have a cs, so remove
@@ -899,8 +898,7 @@
 			 * as a failed response coupled with "observe layer7" caused the
 			 * server state to be suddenly changed.
 			 */
-			conn_sock_drain(conn);
-			cs_close(cs);
+			cs_drain_and_close(cs);
 		}
 
 		if (cs) {