fix several issues related to leaking memory

[rlineweaver]

• remove duplicate decrement on the transaction_pointer during rollback
• remove unnecessary code to handle decrementing the transaction_pointer prior to rollback which caused transactions following the first rollback to fail to decrement the transaction_pointer
• change cleanup() unset the collection instead of reinitializing it to an empty array, so we don't keep numerous empty arrays in memory if the transaction pointer gets off
• add test assertions to ensure that the transaction pointer is always returned to zero and the transaction-specific in-memory objects have been cleared at the end of each test

[gravityrail]

Hi Pat,

We're experiencing these leaks too, specifically when we do a MyModel.create which fails with a validation error.

Could you merge this patch? It fixes the problem.

Thanks, Dan