Fix #260:  break import cycles in datastore w/o factories.

Finishes fix for #260 for datastore (and enforces cycle check in pylint).

Alternative fix to the 'factories' indirection in #268.

Note that this requires adding a messy arg to the 'Entity.from_protobuf'
classmethod, in order to avoid the need to import '_helpers'.

Another way to avoid this cycle would be to make 'Entity.from_protobuf' into a
free function in another module (likely also moving 'Key.from_protobuf' for
the sake of consistency).