[Network] Cleaned NetworkService interface.

addPeer and removePeer methods are no longer declared in the interface but are
now only accessible from the implementation.
[Network] finished HeartbeatThread.

Now on socket I/O errors the PeerSocket update the model throught NetworkService
and exit orderly.
[Network] fixed unicast send to unknown peer.
[Network] fixed some javadoc and log message.
[Network] Improved thread concurrency.

Network classes should now be fully thread-safe and their stop() methods should
be more responsives.
[Network] Refactored some attributes name.