Fix logic in item retriever to avoid requesting stuff we already have.

It used to delete the empty temp request too early, and then recreate it
with a final part (e.g. ENVELOPE) plus RFC822, remove ENVELOPE (because available)
and end up performing a request for RFC822 even though it is available too.

This fixes "Unable to fetch item from backend" errors when offline,
i.e. the inability to read emails while being offline sometimes,
and should also speed things up.

Fixed with Volker's help.