Unfortunately due to a quirk of our googlesource.com implementation,
we can't provide a strong enough consistency guarantee when reading a
change's sub-entities inside of a beginTransaction block, so we need
to provide a hand-written implementation.
For a gwtorm-backed implementation, beginTransaction is likely good
enough, although the actual semantics are still dependent on the
underlying database's isolation level. If this code were going to stay
around for longer and see more production use, we might see more
implementations, but that is almost certainly more trouble than it's
worth.
Change-Id: I27aa35d95b9a4eab3178518065b0cab8823feb4e
ef321bcce5