Eric/inbox feed loader

[EricBAndrews]

[!NOTE] Still in draft while I finalize and bugfix, but the basic architecture is all in place.

Major divergences from the old approach:

• Merging is now done in the Fetcher; this enables much more reuse of the StandardFeedLoader logic
• Child loaders can only have one parent. We never used the multi-parent functionality of the old approach, and having only one avoids a lot of nasty recursion cases