Allow users to choose transaction isolation level?

[jgeewax]

I believe we currently only support the default transaction isolation level for Datastore (snapshot). Could we figure out a way to allow people to choose a serializable transaction?

See https://cloud.google.com/datastore/docs/apis/v1beta2/datasets/beginTransaction

[tseaver]

Connection.begin_transaction already takes a serializable argument: we just need to plumb that through to Transaction.__init__/Transaction.begin.

[tseaver]

... and Client.transaction.