mu: protect against overly long keys

We got some errors when some of the key values exceeded the Xapian
maximum; in particular the message-id.

So make all the key-methods check, and truncate the message-id if
necessary.

lib/mu-msg-iter.cc

@@ -397,10 +397,8 @@ mu_msg_iter_get_thread_id (MuMsgIter *iter)
 	try {
 		const std::string thread_id (
 			iter->cursor().get_document().get_value(MU_MSG_FIELD_ID_THREAD_ID).c_str());
-		
 		return thread_id.empty() ? NULL : g_strdup (thread_id.c_str());
 
-
 	} MU_XAPIAN_CATCH_BLOCK_RETURN (NULL);
 }