Added initial implementation of "imapc" storage.

Added initial implementation of "imapc" storage.
It can be used to create a "smart IMAP proxy" where Dovecot uses remote IMAP
server as a mail storage.

This is a very rough early implementation. Performance isn't good, many
required features are missing, error handling is lacking and code needs
de-uglification. Still, it should be enough for selecting INBOX and
accessing mails in it.