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-api/src/org/briarproject/api/sync/ValidationManager.java

@@ -55,8 +55,9 @@ public interface ValidationManager {
 
 		/**
 		 * Called once for each incoming message that passes validation.
+		 * @return whether or not this message should be shared
 		 */
-		void incomingMessage(Transaction txn, Message m, Metadata meta)
+		boolean incomingMessage(Transaction txn, Message m, Metadata meta)
 				throws DbException;
 	}
 }