Modify archiver's loadFile to allow using stdin.

The archiver spools new database rows to disk files, then flushes those periodically. That uses the COPY from $file command, which is executed by the database server. That does not work if the archiver and the database server are on different hosts.

If we want to fix this, the archiver's loadFile and shutdown methods need to allow running the load as COPY from STDIN. Not hard.