Fix race condition for harvester Start / Stop in registry

[ruflin]

There is still a case where a race condition could happen (see TODO). Registry must be improved to also handle this case.

This should not be backported as the issue can only happen on shutdown in very rare cases and should not have any side effect.

Closes #3779

[7AC]

CI is failing with an error I also got in another review so I think it's unrelated to your changes

ERROR: Test that Filebeat works when stdin is closed.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\gopath\src\github.com\elastic\beats\filebeat\tests\system\test_prospector.py", line 139, in test_stdin_eof
    self.wait_until(lambda: self.output_has(lines=2))
  File "C:\gopath\src\github.com\elastic\beats\filebeat\tests\system\../../../libbeat/tests/system\beat\beat.py", line 283, in wait_until
    "Waited {} seconds.".format(max_timeout))
Exception: Timeout waiting for 'cond' to be true. Waited 10 seconds.
======================================================================

[ruflin]

jenkins, retest it

[ruflin]

Closing in favor of #4314 Too many things have changed in the meantime.