[ZEPPELIN-1754] PING request stacking on websocket reconnect

[pythonmax]

What is this PR for?

Fixes a bug when PING requests are stacking when server restarted.

What type of PR is it?

[Bug Fix]

Todos

No

What is the Jira issue?

[ZEPPELIN-1754]

How should this be tested?

1. build Zeppelin leaving console.log enabled
2. start ZeppelinServer, open any notebook in browser
3. open console with timestamps and check that PING request repeats exactly once in every 10 second
4. restart ZeppelinServer but do not close or refresh the notebook page
5. wait for websocket connection reopened
6. check PING request frequency in console
   PING requests should repeat not often than once per 10 seconds.

Screenshots (if appropriate)

Questions:

• Does the licenses files need update?
  No

• Is there breaking changes for older versions?
  No

• Does this needs documentation?
  No

[marchpig]

I've tested it and works as expected.
LGTM! But please trigger CI again.

[Leemoonsoo]

LGTM and merge to master if there're no more comment.