multi: server connection status

[ellemouton]

Add server connection status info so that a caller of the Server can get informed about changes in the Server's connection. The 3 statuses differ from the Client connection statuses. They are as follows:

• ServerConnStatusNotConnected: the server is not connected to the mailbox. This means it is either the mailbox is down or that the server is busy re-connecting to it.
• ServerConnStatusInUse: A client is currently connected to the server
• ServerConnStatusIdle: the server is connected to the mailbox and ready to accept a connection but there is no client currently connected.

[ellemouton]

thanks @guggero :) updated the names and added a new commit testing both the server and client status in the itests :)