innodb_undo_log_truncate may be blocked if transactions were recovered at startup

The field trx_rseg_t::trx_ref_count that was added in WL#6965 in MySQL 5.7.5 is being incremented twice if a recovered transaction includes both undo log partitions insert_undo and update_undo.

This reference count is being used in trx_purge(), which invokes trx_purge_initiate_truncate() to try to truncate an undo tablespace file. Because of the double-increment, the trx_ref_count would never reach 0.

It is possible that after the failed truncation attempt, the undo tablespace would be disabled for logging any new transactions until the server is restarted (hopefully with a slow shutdown, so that no transactions would be recovered on the next startup).