Fix draft encryption for async crypt in OOM

* src/mail.cpp (Mail::checkSyncCrypto_o): Disable async for
draft encryption.
* src/mailitem-events.cpp: Do not cancel write because crypto
is now done synchronously.

The simple solution to fix draft encryption in the new branch
is not do it asynchronously. Of course we have to block the
write for the draft until we have done the encryption.