Restructured some control flow for CouchDB-ogr2ogr.

The 'i' flag isn't really needed if filetype is converted to lowercase before
pattern matching is done. This removes /^xml$/i.test(filetype) as well. If
Couch-ogr2ogr is installed and an unknown extension is encountered, load_next()
diverts to the block that has no effect for now.
Employed different read modes depending on extension.
WIP: Stored base64-encoded content hierarchy in ogr2ogr_uploads object.