Adding a tracking message to the export actor means it's possible to run them in a precession.
Adding a new precession actor, and some GUI components for triggering exports.
The change also adds a heartbeat to the export process.
Add a check to ensure only documents with "text/html" content type are processed in FeedExporter, AtagExporter, and TermFrequencyExporter. This prevents non-HTML documents from being parsed and helps maintain data consistency and keep the memory usage down.
Since some of the export tasks have been memory hungry, sometimes killing the executor-services, they've been moved to a separate process that can be given a larger Xmx.
While doing this, the ProcessMainClass was given utilities for the boilerplate surrounding receiving mq requests and responding to them, some effort was also put toward making the process boot process a bit more uniform. It's still a bit heterogeneous between different processes, but a bit less so for now.
