director: DIRECTOR-LIST - Show number of USERs sent/received in "handshaking" state

director: Track connections' last ping time

director: Add director_connection_get_status() Can be used to access connection-specific status information.

doveadm director ring status shows now more information

director: Improved logging related to disconnections.

director: Don't try to send the new SYNC parameter to old director versions. This should allow adding new directors to an old director ring without anything breaking. Once all directors have upgraded they start using the new features.

director: Redesigned connection handling and error handling. Director now accepts all connections from everywhere and syncs them until the handshaking is finished. At that point it finally decides if this is a connection that should be used as our left/right connection, or if it should be disconnected. This should make connecting more reliable, especially if one of the directors sends broken handshake or has other trouble.

director: Handle all commands during handshake. Previously the sender might have sent these commands, but the receiver wouldn't have handled them and instead just disconnected.

director: List of director connections belongs to struct director.

director: Changes to PING handling. Use larger ping timeouts. While waiting for sync keep doing rapid pings until the sync is finished, not just once.

director: Make sure all director connections are deinitialized at exit. --HG-- branch : HEAD

director: Lots of fixes. It should be pretty stable now. --HG-- branch : HEAD

director: A lot of fixes. --HG-- branch : HEAD

Added initial implementation of a director process (for NFS users). There are still some unimplemented features and bugs. Also changing mail server list doesn't yet make sure that other directors won't assign the same user to a different server at the same time. --HG-- branch : HEAD