MDEV-17243: "FSM: no such a transition ABORTING -> REPLICATING"

[janlindstrom]

TrxMap structure doesn't take into consideration presence of two trx
objects with same trx_id (2^64 - 1 which is default trx_id) belonging
to two different connections.

This eventually causes same trx object to get shared among two
different unrelated connections which causes state inconsistency
leading to crash (RACE CONDITION).

This problem could be solved by taking into consideration conn-id,
but that would invite interface change. To avoid this we should
maintain a separate map of such trx objects based on gu_thread_id.

https://jira.mariadb.org/browse/MDEV-17243 and
https://jira.mariadb.org/browse/MDEV-17262

[temeo]

This looks like a suspicious patch for two reasons:

• Application which is using Galera library (MariaDB in this case) should guarantee that it assigns unique trx_id for each transaction before calling wsrep hooks. Therefore the transaction with trx_id = -1 (undefined trx id in MariaDB) should never end up in connection map
• gu_thread_t is used as a workaround for transactions with invalid trx id, but this assumes that the same thread always handles the whole transaction cycle, which may not hold in all cases.