Reduced memory leak problem in transactions by lazily updating AR objects

This pull request concerns Issue https://github.com/rails/rails/issues/776. 

I handle memory bloat by having the transaction hold only the AR objects which it absolutely needs to know about. These are the AR objects with callbacks (they need to be updated as soon as something in the transaction occurs). 

All other AR objects can be updated lazily by keeping a reference to a TransactionState object. If an AR object is inside a transaction, then the transaction will add its TransactionState to the AR object. When the user makes a call to some attribute on an AR object (which has no callbacks) associated with a transaction, the AR object will call the sync_with_transaction_state method and make sure it is up to date with the transaction. After it has synced with the transaction state, the AR object will return the attribute that was requested.

Most of the logic in the changes are used to handle multiple transactions, in which case the AR object has to recursively follow parent pointers of TransactionState objects.