fix(healthcheck) remove event watcher when stopping hc (#74)

Co-authored-by: Brian Fox <brianhfox@gmail.com>

Co-authored-by: Brian Fox <brianhfox@gmail.com>

lib/resty/healthcheck.lua

@@ -1141,6 +1141,7 @@ end
 function checker:stop()
   self.checks.active.healthy.active = false
   self.checks.active.unhealthy.active = false
+  worker_events.unregister(self.ev_callback, self.EVENT_SOURCE)
   self:log(DEBUG, "healthchecker stopped")
   return true
 end
@@ -1441,7 +1442,6 @@ function _M.new(opts)
       -- just a wrapper to be able to access `self` as a closure
       return self:event_handler(event, data.ip, data.port, data.hostname)
     end
-    worker_events.register_weak(self.ev_callback, self.EVENT_SOURCE)
 
     -- handle events to sync up in case there was a change by another worker
     worker_events:poll()