Initial framework for direct transfer of backups to remote storage.

Add a layer of indirection in the writing of files to the backup store, and
create a background thread to handle the processing of files to be stored.
Right now this secondary thread does not do much, but will easily be able
to launch a helper script for transferring data to a remote server.

Files are processed by the background thread one at a time.  Multiple files
can be queued up for processing, but the size of the queue is limited so
that the production of backup data will be throttled to the speed at which
the data can be transferred (to bound the temporary space needed for
storing files).