Couchstore compaction creates and then overwrites header

When you try to open a couchstore database (for creation) it'll start off by writing a header block at offset 0. That database header isn't really that interesting as there isn't any data in the database yet. As part of compaction we want to get rid of all unused space in the file, and this block doesn't contain any data so we want to get rid of it. This is currently done by overwriting it. Instead, header creation should be delayed until the first commit.