Improved handling of timing functions for fetching train data

[mattdy]

I've been having problems where this module becomes stuck at a particular time and no longer tries to fetch any updates. I narrowed this down to the case where a request for GET_TRAININFO was sent, but due to my internet connectivity being down at the time, this request would never get handled and answered with a TRAIN_DATA reply. Because the next call to scheduleUpdate() only occurred after the reply is handled, this would cause the module to get stuck and never update.

This PR replaces the scheduleUpdate() functionality with an initial setTimeout() call in start() (delaying the first request according to the this.config.initialLoadDelay parameter), and then fetching data with setInterval() at the appropriate timing from this.config.updateInterval.

As this loop doesn't rely on the data actually being received and processed, it will mean any dropped requests do not result in the module becoming frozen.