This introduces synchronized literals into the IMAP stack.  This
work also includes a new Memory.Buffer interface (replacing the old
Memory.AbstractBuffer classes) which is more flexible and has more
potential for avoid buffer copies.  Some work in RFC822 and Db use
these new unowned buffer capabilities.

This opens the door for saving drafts (#6992).  It also fixes issues
people have reported using UTF-8 usernames and passwords with IMAP.