Optimization for gRPC streams throughput.

gRPC internal logic marshalls the protobuf response payloads sequentially before sending them over the wire (see io.grpc.ServerCallImpl.sendMessageInternal), imposing as limit the maximum marshalling throughput of a payload type.

We've observed empirically that ScalaPB-generated message classes have associated marshallers with significant latencies when encoding complex payloads (e.g. com.daml.ledger.api.v1.transaction_service.GetTransactionTreesResponse), with the gRPC marshalling bottleneck appearing in some performance tests.

To alleviate the problem, we can leverage the fact that ScalaPB message classes have the serializedSize value memoized, (see scalapb.GeneratedMessage.writeTo(output:java\.io\.OutputStream)*), whose computation is roughly half of the entire marshalling step.

This optimization method takes advantage of the memoized value and forces the message's serializedSize computation, roughly doubling the maximum theoretical ScalaPB stream throughput over the gRPC server layer.