Need to debounce pending writes into batches

[Aaronontheweb]

The initial performance numbers were about 1/5-1/6 of what DotNetty can do, and one of the reasons why I believe this is the case is that gRPC performs a flush on each write to the stream - and you're required to wait for that flush to complete prior to sending the next message. Thus we have a flow control problem that eats up an enormous amount of CPU and destroys throughput.

We need to do what we did in DotNetty a long time ago and add something similar to our https://github.com/akkadotnet/akka.net/blob/dev/src/core/Akka.Remote/Transport/DotNetty/BatchWriter.cs - which is able to group pending writes together into a continuous (but frame-length encoded) chunk that gets flushed as soon as the transport is ready again.

I think we can do this by simply modifying our Protobuf message definitions to contain arrays of ByteString rather than a single payload - and we can use semantics similar to what the BatchWriter does.

[Aaronontheweb]

Resolved mostly via #7