Per-trigger statistics and MQTT improvements

Breaking changes

• statusTopic can no longer be set on the mqtt configuration in settings.json. Overall status messages are always sent to the node-deepstackai-trigger/status topic. This change aligns the status topic messages with the new MQTT messages that the system listens to for resetting statistics.

Non-breaking changes

• Per-trigger statistics are now sent in new MQTT messages published to the node-deepstackai-trigger/statistics topic. The trigger name, triggered count and analyzed file count are included, as well as a formatted string version suitable for presentation to a user. The per-trigger statistics are also available as variables for mustache templates. Resolves issue 306.
• Statistics can be reset by publishing specific MQTT messages to the node-deepstackai-trigger/statistics topics and sub-topics. Resolves issue 308.
• Statistics can be read and reset via new REST APIs. Overall statistics are available at /statistics and per-trigger statistics are available at /statistics/triggerName. Statistics for all triggers can be retrieved from /statistics/all. See the API documentation for more information. Resolves issue 307 and issue 311.
• Triggers can be activated by sending node-deepstackai-trigger/motion MQTT messages with the name of the trigger in the message. This is similar to activating a trigger via the REST API and results in the trigger attempting to download a snapshot from the snapshotUri specified in the trigger's configuration. Resolves issue 314.
• Shutting down the system after a failed launch no longer throws an error. Resolves issue 301.
• The underlying Linux variant used for the Docker image is now node:slim. Resolves issue 299.
• Pushbullet is now included in the settings JSON schema. Resolves issue 316.