Now that we have mostly successfully implemented an alternate
storage backend for files data, let's start to define the
interface for it!
This commit takes the mostly-working interface as defined by the
simple store repo and codifies it as the file storage interface.
The interface has been split into its logical components:
- index metadata
- fulltext data
- everything else
I don't consider the existing interface to be great. But it will
help to have it more formally defined so we can start chipping away
at refactoring it.