[5.3] dont rollback to savepoints on deadlock

[halaei]

This fix only applies to calls to DB::transaction() inside another transaction. e.g.:

\DB::transaction(function(){
    ...
    //directly or indirectly call another transaction:
    \DB::transaction(function() {
        ...
        //dead lock!
        ...
    }, 2);//attempt twice
}, 2);//attempt twice

Deadlock in the inner transaction() function means the DBMS has rolled back the outer transaction as well. So the inner transaction function should not attempt again. It only should decrement Connection::transactions before throwing exception, so that the outer transaction() function attempts again.

Here is a quote from MySQL Reference Manual:

A transaction deadlock causes InnoDB to roll back the entire transaction. Retry the whole transaction when this happens.

[taylorotwell]

Needs a full description of what this fixes, etc.