Catch gear.InterruptedError

This is how gear lets us know that it has been stopped by another
thread.  Since we weren't catching these exceptions explicitly,
they were polluting the logs on shutdown (especially in tests).
Catch them and return immediately from the daemon run methods.

Change-Id: I6f845566c759651e630d7f93c94dce975ad53d30

zuul/merger/server.py

@@ -100,6 +100,8 @@ class MergeServer(object):
                 except Exception:
                     self.log.exception("Exception while running job")
                     job.sendWorkException(traceback.format_exc())
+            except gear.InterruptedError:
+                return
             except Exception:
                 self.log.exception("Exception while getting job")