(PDB-812) Consolidate produce-body

This patch consolidates produce-body functions for all streaming endpoints to a
new function in http/produce-body.
@kbarber this is split into a new file to avoid circular dependencies that arise due to references between the http files and query_eng.clj (which seems like the logical place.)  I'm still experimenting with collapsing the http files affected by this patch into a single namespace, but I'm starting to think that's going to be a dud.