Writing backup metafile incrementally

As discussed on the mailing list, we can save a chunk of memory by writing the backup metafile during the course of the backup, rather than storing the set of files backed up in memory till the end.

The main wrinkle here is that we still have to spool entries if we have a CompositeChunk open, because we can't record the metafile entry without the chunk checksum.

Also, this patch breaks the t/03-combine-little-files.t test, because that uses the set of saved files explicitly in the test. I've got another patch which keeps the old behaviour if specifically requested, if we want to handle this that way.

-G