Storcli/storaged TCP connection supervision (detection time less than 5s)

  - add an applicative callback on each af_inet connection for TCP connection supervision:
    The application can probe the other end by sending a NULL rpc procedure
    The probing is started upon the expiration of a pre-configure timer that supervises
    the response from a storaged (default is 3 sec)

  - add a callback to the af_inet connection in order to inform the application about
    the TCP connection availability. By this way a LBG can compute is global availibility

 - add a application probing at LBG level on the same model as the af_inet connection:
    The probing is starting upon a time-out detection on the LBG. The LBG becomes
    unselectable until the LBG gets a positive answer on any of the TCP connection
    under its control.