default for checkpoint txn interval is too low

The default checkpoint interval is currently set to 40k transactions. That's way too low (I don't know what idiot set it to that.. oh wait, it was me...)

The old default in 1.0 is 64MB. Assuming an average of 100 bytes per txn, we should have the txn-count based interval default to at least 640,000. I'd like to change to 1M as a nice round number.