enforce_autocommit_on_reads

[renecannao]

WHY

Since 3449ab0 ( #438 ) ProxySQL tracks the value of autocommit and enforces it in database connection.
This implementation had the drawback that if a client sends set autocommit=0 , and read/write split is implemented too, transactions are opened on slaves as pointed in #469 , and therefore the feature was reverted.
Now ProxySQL doesn't track anymore the value of autocommit, and this means that statements could be executed in autocommit mode when the client assumes that autocommit is OFF .
This is problematic for application and libraries that do not explicitly starts transactions. See:

• http://docs.sqlalchemy.org/en/latest/core/connections.html#understanding-autocommit
• https://www.python.org/dev/peps/pep-0249/#commit

WHAT

• add a new variable mysql-enforce_autocommit_on_reads
• if the variable is set to false (default) and a SELECT (not FOR UPDATE) is executed, the client value of autocommit is not enforced into the database connection
• in all the other cases, the client value of autocommitenforced into the database connection