ryan <ryanb+appeng...@google.com>

hi david! you're in roughly the same neighborhood, but there are a
number of key differences between what you've outline and what the
datastore actually does. here are a few:

- each entry in an entity group's tx log has a single timestamp. there
aren't separate separate read and last committed timestamps.
- we only fail at commit time, not earlier. specifically, we never
consider a read "invalid" just because a write has happened since the
tx started. we just read the data as of when the tx started, ie before
the new write. if a write occurs within the tx, we'll fail on commit.
if the tx is read-only, then there's no need to fail.
- when starting a tx, its timestamp is always generated based on the
last committed timestamp of the entity group. we can't just use the
time on the local server, since clocks aren't perfectly synchronized
across servers.
- note that txes may not be retried purely within the datastore, since
writes may depend on the values of earlier reads. if the entities that
were read have changed, we must re-run your application logic to
recalculate the derived writes. that's why run_in_transaction() takes
a lambda, so that we can re-run it if necessary.