Only allow one sync read to happen at a time.

[s0undt3ch]

What does this PR do?

Makes sure that only one sync read happens at a time, thus avoiding:

Traceback (most recent call last):
  File "/var/lib/jenkins/.pyenv/versions/2.7.9/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/var/lib/jenkins/workspace/raas/pr/1275/.virtualenvs/Salt-Py2.7.9/lib/python2.7/site-packages/salt/utils/process.py", line 613, in _run
    return self._original_run()
  File "/var/lib/jenkins/workspace/raas/pr/1275/.virtualenvs/Salt-Py2.7.9/lib/python2.7/site-packages/salt/utils/event.py", line 1092, in run
    for event in events:
  File "/var/lib/jenkins/workspace/raas/pr/1275/.virtualenvs/Salt-Py2.7.9/lib/python2.7/site-packages/salt/utils/event.py", line 620, in iter_events
    data = self.get_event(tag=tag, full=full, match_type=match_type)
  File "/var/lib/jenkins/workspace/raas/pr/1275/.virtualenvs/Salt-Py2.7.9/lib/python2.7/site-packages/salt/utils/event.py", line 579, in get_event
    ret = self._get_event(wait, tag, match_func, no_block)
  File "/var/lib/jenkins/workspace/raas/pr/1275/.virtualenvs/Salt-Py2.7.9/lib/python2.7/site-packages/salt/utils/event.py", line 484, in _get_event
    raw = self.subscriber.read_sync(timeout=wait)
  File "/var/lib/jenkins/workspace/raas/pr/1275/.virtualenvs/Salt-Py2.7.9/lib/python2.7/site-packages/salt/transport/ipc.py", line 664, in read_sync
    return ret_future.result()
  File "/var/lib/jenkins/workspace/raas/pr/1275/.virtualenvs/Salt-Py2.7.9/lib/python2.7/site-packages/tornado/concurrent.py", line 237, in result
    raise_exc_info(self._exc_info)
  File "/var/lib/jenkins/workspace/raas/pr/1275/.virtualenvs/Salt-Py2.7.9/lib/python2.7/site-packages/tornado/gen.py", line 285, in wrapper
    yielded = next(result)
  File "/var/lib/jenkins/workspace/raas/pr/1275/.virtualenvs/Salt-Py2.7.9/lib/python2.7/site-packages/salt/transport/ipc.py", line 641, in _read_sync
    raise exc_to_raise  # pylint: disable=E0702
AssertionError: Already reading
Process Reactor-7:

Previous Behavior

The above exception would be shown multiple times

Tests written?

No

[cachedout]

We should also run through this on Py3 before we get it in.

[grep4linux]

I tried the fix and the salt * state.highstate does not return.

[cachedout]

@grep4linux Can you give us any more information? Debug logs, output, etc?