pool,ftp: do not query for checksum on write

pool fetches checksum on write, even if in most cases it's not defined.
From now on, the pool fetches the  checksum only if it's not defined.
Doors will always define a checksum ( possibly empty ).

Acked-by: Paul Millar
Acked-by: Gerd Behrmann
Target: master
Require-book: no
Require-notes: no