txn: completely reset group state on term change

When the consumer group log's term change we replay the whole log to
reconstruct the state. We used to merge current and the replayed state
but it's error prone. Reseting the whole txn state to have more deter-
ministic behavior

src/v/kafka/server/group.cc

@@ -1672,6 +1672,10 @@ void group::reset_tx_state(model::term_id term) {
     // to avoid modifying the state of the executing tx methods
     _term = term;
     _volatile_txs.clear();
+    _prepared_txs.clear();
+    _expiration_info.clear();
+    _tx_data.clear();
+    _fence_pid_epoch.clear();
 }
 
 void group::insert_prepared(prepared_tx tx) {