This configuration can be passed to most datastore calls using the
Optional deadline; default None (which means the
system default deadline will be used, typically 5 seconds).
Optional callback function; default None. If
specified, it will be called with a UserRPC object as argument
when an RPC completes.
Optional read policy; set to EVENTUAL_CONSISTENCY to
enable eventually consistent reads (i.e. reads that may be
satisfied from an older version of the datastore in some cases).
The default read policy may have to wait until in-flight
transactions are committed.
Other keyword arguments as long as they are supported by