Cleanup Rdb_transaction::m_is_tx_flag

[laurynas-biveinis]

• The flag does not mark the failure of the whole transaction, but rather whether the previous statement was rolled back. Rename the field to m_was_last_stmt_rolledback accordingly, rename its setter set_tx_failed to set_last_stmt_rollback, make the field private, introduce a getter was_last_stmt_rolledback, add it to the class invariant as much as possible.
• Rename commit_or_rollback method to commit_or_rollback_autocommit_trx
• Extract a private helper on_finish, which is called from on_commit and on_rollback.
• Introduce an abstract virtual method start_autocommit_stmt
• Remove an outdated comment from ha_rocksdb::external_lock

[laurynas-biveinis]

This fell out from debugging range locking snapshot ignore start and stop points for DML statements, and will be a dependency PR for the range locking.

[laurynas-biveinis]

Havin slept on this, I don't like that the flag is set for two cases but read only for one of them. Let's try reducing its setting to the one case, and maybe the reset code will become simpler too.

[laurynas-biveinis]

Even better, it should be possible to remove the flag altogether

[laurynas-biveinis]

Superseded by https://github.com/facebook/mysql-5.6/pull/1453