[Dovecot] dsync replication available for testing

[Timo Sirainen]

In dovecot-2.1 hg you can now test dsync-based replication. Everything 
isn't finished yet, but it appears to work and I've enabled it for my 
@dovecot.fi mails. Some issues:

  - public namespace isn't replicated at all
  - shared namespace is replicated, but not private mail flags
  - I've only tested SSH replication setup now, not director replication 
setup (and director setup is still missing many things)
  - SSH replication setup uses aggregator process, which isn't really 
necessary and can probably be avoided in future

Below is a configuration for virtual user setup. System user 
configuration works pretty much the same, except doveadm/ssh is run as root.

Try first that dsync works successfully with ssh in host1:

doveadm sync -u user at domain remote:vmail at host2.example.com

and also in host2:

doveadm sync -u user at domain remote:vmail at host1.example.com

------

mail_plugins = $mail_plugins notify replication

service aggregator {
   # give enough permissions for mail processes
   fifo_listener replication-notify-fifo {
     user = vmail
     mode = 0600
   }
   unix_listener replication-notify {
     user = vmail
     mode = 0600
   }
}

service replicator {
   # start replication at startup
   process_min_avail = 1
}

plugin {
   # host1 replicates to host2
   mail_replica = remote:vmail at host2.example.com
   # host2 replicates to host1
   #mail_replica = remote:vmail at host1.example.com
}

#dsync_remote_cmd = ssh -l%{login} %{host} doveadm dsync-server -u%u 
-l%{lock_timeout} -n%{namespace}

service doveadm {
   # if you're using a single virtual user, set this to
   # start ssh as vmail (not root)
   user = vmail
}