don't mark data as dirty, just write it

it was too easy for an open file to get out of sync with the copy on
webhdfs.  use the checksums to avoid copying down unnecessarily, but
let's always write the whole file to webhdfs on write/truncate.
bring dirty back

uploading with every write was just too slow.  so bring dirty flag back,
and in case of collision (server updated, but we have dirty local data),
the server wins.