Switch akka-http to just exposing a stream for file uploads instead of buffering to disk

[blast-hardcheese]

Semantics need to be addressed, but:

• http4s and dropwizard both do streaming file uploads, not disk-backed
• multi-file uploads for a single endpoint are rare use cases, maybe can be supported through special opt-in
• mandated, per-route file handling methods are cumbersome

One issue is that if files are not the final element in the payload, (multipart form-data, for instance), we will run into a situation where we cannot call the application code before the stream has been consumed. Currently this is handled by filesystem-backed buffer, so an attempt to change this without a transition period will cause currently functional requests to start failing. This should be handled delicately, either by an option with a deprecation cycle, at the very least migration notes that go along with the new function signatures.