[feature] 'set autocommit=0' auto txn begin

[BohuTANG]

Summary

In TPCC, the client gives:

set autocommit=0

treats as txn begin, it maybe commit/rollback by tpcc.
We should do txn begin when client give the set query set autocommit=0

[andyli029]

References:

be related to: proxysql
proxysql does not treat autocommit=0 as a transaction
AUTOCOMMIT=0 Transaction Pattern not working on ProxySQL 1.4

from proxysql:

autocommit=0 doesn't start a transaction, therefore proxysql 
"not treating autocommit=0 as a transaction" is technically correct, 
because there is no transaction yet.

Added new variable mysql-autocommit_false_is_transaction

If mysql-autocommit_false_is_transaction=true (false by default), 
a connection with autocommit=0 is treated as a transaction.

If forward_autocommit=true (false by default), the same behavior applies.