Separate ingest from query

[jdegoes]

The goal of this ticket is to ensure that ingest and query are completely decoupled and interact only through the file system, so that overall architectural complexity is minimized and other tickets may be completed more easily.

1. Ingest must write data to a file system.
2. Query must read data from a file system.
3. Query and ingest may interact only through a file system.

This is mostly (entirely?) already the case, but we need to verify the extent of the separation and severe any remaining ties that exist.