[mms] Remove read/write buffering from stream connection to remote server.
[mms] So many IMAP servers have issues with BINARY, that we should not send literal8 data unless we absolutely have to.

We lose the benefit of not having to scan the outgoing data, but APPEND
actions are farily rare so we need to bite the bullet and do the check
client-side since server-side is so unpredictable.
Bump import limit

Turns out that this is widely IMAP server dependent.  Old UW-IMAP can
import messages in a few seconds.  Newer versions of Dovecot (2.1 - 2.2)
take FOREVER (maybe due to indexing?).  Since Dovecot is used by more
than 50% of installations, we do need *some* sort of limit. And we
absolutely have to prevent users from being able to import mailboxes
with 100,000 messages since that is never going to work consistently.
Optimize emptying the trash mailbox.