Sensors fixes for state, restart configurations, death trigger emit

[guzzijones]

• Added a new internal core.st2.sensor.process_abandoned trigger which is emitted when the sensor
  container permanently gives up on a sensor after exceeding the maximum number of respawn attempts.
  This lets users define their own alerting (email, Slack, workflow, etc.) via a rule instead of a
  sensor failing silently.
• Added a SensorInstanceDB collection tracking each sensor's runtime health (status, hostname, pid,
  exit code, respawn count, last updated). The health fields are merged into the /v1/sensors API
  response and shown by st2 sensor list / st2 sensor get, and can be filtered with
  st2 sensor list --status=abandoned.
• Added [sensorcontainer].max_respawn_count, [sensorcontainer].respawn_delay and
  [sensorcontainer].respawn_backoff_factor config options to control how many times a crashed
  sensor is respawned before being abandoned and how long to wait between attempts. The backoff
  factor defaults to 1 (a constant respawn_delay between attempts); a value greater than 1
  grows the delay exponentially.