Update sensors to store temporary data in the datastore

This pull request utilizes new feature from https://github.com/StackStorm/st2/pull/934.

It updates `TwitterSearchSensor` and `GithubRepositorySensor` to retrieve and persist last_id in the datastore.

This allows sensors to resume from where the were left if a sensor container process is restarted or if a sensor crashes. This means no more (annoying) duplicates events on restarts.

Note: This PR is fully backward compatible with current stable version of StackStorm - if new functionality is not available, we simply fall back to the old behavior.