Allow Validator to access metadata for pending messages

Database queries for metadata only returned it for messages that were delivered already.
However, there are cases (e.g. a pending message needs to be delivered) where
the validator needs to retrieve the metadata from the database.

For these cases, a special database query has been introduced.

briar-tests/src/org/briarproject/db/H2DatabaseTest.java

@@ -1006,6 +1006,10 @@ public class H2DatabaseTest extends BriarTestCase {
 		map = db.getMessageMetadata(txn, groupId);
 		assertTrue(map.isEmpty());
 
+		// validator gets also metadata for pending messages
+		retrieved = db.getMessageMetadataForValidator(txn, messageId);
+		assertFalse(retrieved.isEmpty());
+
 		db.commitTransaction(txn);
 		db.close();
 	}