atom: switch to getline/close for response bodies

atom: switch to getline/close for response bodies

This will let us stream larger Atom documents bodies without
wasting too much memory and reduce the amount of round-trip
requests needed to get necessary information.

Hopefully clients are using streaming (SAX) parsers, too.

This is the final transition in the core public-inbox
code to allow migrating to a "pull"-based body streaming
scheme which allows a HTTP server to respond appropriately
to backpressure from slow clients.