Upgrade Bigtable dependency to 0.9.6.2

Cloud Bigtable 0.9.6.2 has some fixes relating to:

1) Using dependencies for GCP protobuf objects rather than including generated artifacts directly in bigtable-protos
2) BulkMutation bug fixes
3) Auth token management
4) Using fewer grpc experimental features.

All are important in the context of beam, so the beam dependency should be upgraded.

One snag came up. BigtableSession.isAlpnProviderEnabled() was removed in order to reduce the number of grpc experimental features. BigtableServiceImpl.tableExists() can no longer depend on isAlpnProviderEnabled().