We can slightly optimize field lookups for change streams in the ChangeStreamUnwindTransactionStage.

In TransactionOpIterator::_assertExpectedTransactionEventFormat(), we should not use ValueComparator::kInstance.evaluate(), but compare the string value returned by getStringData() against a StringData constant.

Additionally, we can optimize away the lookup of the o field here in case the operation is not a command. We can instead add an early exit for this case, without looking up the o field at all.