When a message is shared, share its transitive dependencies

Like other recursive operations on the dependency graph, this is
not done in a single transaction to prevent an attacker from creating
arbitrary large transactions.

So at startup, the `ValidationManager` finds and resumes any
unfinished operations, by looking for shared messages with unshared
dependencies.

briar-core/src/org/briarproject/messaging/MessagingManagerImpl.java

@@ -93,7 +93,7 @@ class MessagingManagerImpl extends BdfIncomingMessageHook
 	}
 
 	@Override
-	protected void incomingMessage(Transaction txn, Message m, BdfList body,
+	protected boolean incomingMessage(Transaction txn, Message m, BdfList body,
 			BdfDictionary meta) throws DbException, FormatException {
 
 		GroupId groupId = m.getGroupId();
@@ -106,6 +106,9 @@ class MessagingManagerImpl extends BdfIncomingMessageHook
 		PrivateMessageReceivedEvent event = new PrivateMessageReceivedEvent(
 				header, groupId);
 		txn.attach(event);
+
+		// don't share message
+		return false;
 	}
 
 	@Override