Python Multicast discovery: Use a 3-step discovery and delayed notif'

- Added registerDelayed() to the core directory: it allows to register a peer without notifying peer listeners immediately

Discovery process:
1. A sends a heart beat, containing its HTTP port & path
2. B sends a "step 1" message using the HTTP transport implementation, using UDP packet information and content (UDP sender IP, heart beat port and path). This message contains the peer dump of B.
3. A registers the B, but doesn't notify its listeners. It then sends a "step 2" message to B, with its dump
4. B registers A, notifies its listeners, then sends an empty "step 3" message
5. A notifies its listeners about B
Java implementation of the 3-steps multicast discovery
RPC Discovery: on registered, send "contact" to the new peer only

... instead of all peers
New servlet checkAccess() policy

- If the peer UID doesn't match the known HTTP port: force the message sender UID to "<invalid>"
- If the peer UID is unknown, keep the one used in the request
Java uses the same export configuration as Python (herald-jabsorbrpc)
Added missing Import-Package: org.json
Corrected type-check in Exporter: ERROR

In the current state, an UnmarshallException is thrown.