Eventlet causing issues in thread pools for sensors

See https://github.com/eventlet/eventlet/issues/147

Observed with msexchange pack, which uses requests when watching the sensor container directly.

[root@irtyrt1 msexchange]# /opt/stackstorm/st2/bin/st2sensorcontainer --config-file=/etc/st2/st2.conf --sensor-ref=msexchange.ItemSensor
2017-10-30 03:52:44,287 DEBUG [-] Using config files: /etc/st2/st2.conf
2017-10-30 03:52:44,287 DEBUG [-] Using logging config: /etc/st2/logging.sensorcontainer.conf
2017-10-30 03:52:44,289 INFO [-] Connecting to database "st2" @ "127.0.0.1:27017" as user "stackstorm".
2017-10-30 03:52:44,454 INFO [-] Setting up container to run 1 sensors.
2017-10-30 03:52:44,454 INFO [-]  Sensors list - [u'msexchange.ItemSensor'].
2017-10-30 03:52:44,455 INFO [-] (PID:25913) SensorContainer started.
2017-10-30 03:52:44,455 INFO [-] Running sensor msexchange.ItemSensor
2017-10-30 03:52:44,466 INFO [-] Connected to amqp://guest:**@127.0.0.1:5672//
2017-10-30 03:52:44,466 AUDIT [-] Access granted to "sensors_container" with the token set to expire at "2017-10-31T07:52:44.464372Z". (username='sensors_container',token_expiration='2017-10-31T07:52:44.464372Z')
2017-10-30 03:52:44,467 WARNING [-] "auth.api_url" configuration option is not configured
2017-10-30 03:52:44,504 INFO [-] Sensor msexchange.ItemSensor started
2017-10-30 03:52:45,401 INFO [-] Found config for sensor "ItemSensor"
2017-10-30 03:52:45,411 INFO [-] Watcher started
2017-10-30 03:52:45,411 INFO [-] Running sensor initialization code
2017-10-30 03:52:45,431 INFO [-] Connected to amqp://guest:**@127.0.0.1:5672//
2017-10-30 03:52:47,016 INFO [-] API version "Exchange2007" worked but server reports version "Exchange2016". Using "Exchange2016"
2017-10-30 03:52:47,019 ERROR [-] Exception in thread Thread-17:
Traceback (most recent call last):
  File "/usr/lib64/python2.7/threading.py", line 812, in __bootstrap_inner
    self.run()
  File "/usr/lib64/python2.7/threading.py", line 765, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/usr/lib64/python2.7/multiprocessing/pool.py", line 301, in _handle_workers
    while thread._state == RUN or (pool._cache and thread._state != TERMINATE):
AttributeError: '_MainThread' object has no attribute '_state'


2017-10-30 03:52:47,101 INFO [-] Running sensor in passive mode
2017-10-30 03:52:47,101 AUDIT [-] Creating new Client object.
2017-10-30 03:52:47,106 AUDIT [-] Access granted to "sensor_service" with the token set to expire at "2017-10-31T07:52:47.104604Z". (username='sensor_service',token_expiration='2017-10-31T07:52:47.104604Z')
2017-10-30 03:52:47,106 WARNING [-] "auth.api_url" configuration option is not configured
2017-10-30 03:52:47,106 AUDIT [-] Retrieving value from the datastore (name=msexchange.ItemSensor:exchange.item_sensor_date_str)
2017-10-30 03:52:47,130 ERROR [-] /opt/stackstorm/virtualenvs/msexchange/lib/python2.7/site-packages/exchangelib/folders.py:127: UserWarning: The get_folder_by_name() method is deprecated. Use "[f for f in self.walk() if f.name == name]" or "some_folder / 'Sub Folder'" instead, to find folders by name.
  warnings.warn('The get_folder_by_name() method is deprecated. Use "[f for f in self.walk() if f.name == name]" '

2017-10-30 03:52:47,585 INFO [-] Found 0 items
2017-10-30 03:52:52,668 AUDIT [-] Retrieving value from the datastore (name=msexchange.ItemSensor:exchange.item_sensor_date_str)
2017-10-30 03:52:52,819 INFO [-] Found 0 items
2017-10-30 03:52:57,901 AUDIT [-] Retrieving value from the datastore (name=msexchange.ItemSensor:exchange.item_sensor_date_str)
2017-10-30 03:52:58,042 INFO [-] Found 0 items
2017-10-30 03:53:03,125 AUDIT [-] Retrieving value from the datastore (name=msexchange.ItemSensor:exchange.item_sensor_date_str)
2017-10-30 03:53:03,272 INFO [-] Found 0 items
2017-10-30 03:53:08,352 AUDIT [-] Retrieving value from the datastore (name=msexchange.ItemSensor:exchange.item_sensor_date_str)
2017-10-30 03:53:08,498 INFO [-] Found 0 items
2017-10-30 03:53:13,578 AUDIT [-] Retrieving value from the datastore (name=msexchange.ItemSensor:exchange.item_sensor_date_str)
2017-10-30 03:53:13,719 INFO [-] Found 0 items
2017-10-30 03:53:18,800 AUDIT [-] Retrieving value from the datastore (name=msexchange.ItemSensor:exchange.item_sensor_date_str)
2017-10-30 03:53:18,942 INFO [-] Found 0 items
2017-10-30 03:53:24,025 AUDIT [-] Retrieving value from the datastore (name=msexchange.ItemSensor:exchange.item_sensor_date_str)
2017-10-30 03:53:24,166 INFO [-] Found 0 items
2017-10-30 03:53:29,246 AUDIT [-] Retrieving value from the datastore (name=msexchange.ItemSensor:exchange.item_sensor_date_str)
2017-10-30 03:53:29,399 INFO [-] Found 0 items
2017-10-30 03:53:34,480 AUDIT [-] Retrieving value from the datastore (name=msexchange.ItemSensor:exchange.item_sensor_date_str)
2017-10-30 03:53:34,622 INFO [-] Found 0 items
2017-10-30 03:53:39,703 AUDIT [-] Retrieving value from the datastore (name=msexchange.ItemSensor:exchange.item_sensor_date_str)
2017-10-30 03:53:39,844 INFO [-] Found 0 items
2017-10-30 03:53:44,933 AUDIT [-] Retrieving value from the datastore (name=msexchange.ItemSensor:exchange.item_sensor_date_str)
2017-10-30 03:53:45,076 INFO [-] Found 1 items
2017-10-30 03:53:45,157 ERROR [-] Exception in thread Thread-18:
Traceback (most recent call last):
  File "/usr/lib64/python2.7/threading.py", line 812, in __bootstrap_inner
    self.run()
  File "/usr/lib64/python2.7/threading.py", line 765, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/usr/lib64/python2.7/multiprocessing/pool.py", line 315, in _handle_tasks
    if thread._state:
AttributeError: '_MainThread' object has no attribute '_state'

Start at the st2sensorcontainer entry point and inspect how sensors using requests are run with Eventlet and multiprocessing thread pools. Reproduce the failure with the shown sensor-container command, then compare the behavior with Eventlet issue 147. Done means the thread-pool worker threads no longer raise the reported _MainThread _state errors while the sensor continues polling.