Add stream splits gRPC.

[fmassot]

Add the stream_splits gRPC method and remove the list_splits gRPC method.

I found it a bit more convenient to add alist_splits method in the MetastoreServiceExt trait rather than enriching the service stream response by adding some helpers to pump the stream and deserialize the splits.

TODO:

• Fix tests.
• Add a test with thousands of splits
• Run the metastore test suite on the grpc client
• Reorganize metastore test suite