http: fix _dump regression

A recent set of changes removed _consuming tracking from server incoming messages which ensures that IncomingMessage#_dump only runs if the user has never attempted to read the incoming data. Fix by reintroducing _consuming which tracks whether IncomingMessage#_read was ever successfully called.

Long-term it would be nice if it was possible to reply with status code 413 to a request with a long payload (without actually handling all of that payload) and not have the http client throw write EPIPE error while not receiving any of that response.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines