Keep the nats watch alive and heard: recover a faulted look, beat after every look, read only what closed since (hq issue 270, review 274)

A watch that panicked stopped for good, and one that could not read said so
only to stderr, so a dead watch read as no slow consumers. A faulted look is
now recovered and the loop goes on; every look ends with a bus.watch
heartbeat saying whether the watch is healthy, which the controller raises
when it stops or says it cannot see. The record is read newest first back to
the last look, and closures that could not be published are forgotten after
the hour.

modules/nats/module.json

@@ -26,7 +26,8 @@
   ],
   "emits": [
     "bus.slow-consumer",
-    "bus.connection-closed"
+    "bus.connection-closed",
+    "bus.watch"
   ],
   "consumes": [],
   "listens": [