Introduce multiplexed grpc-over-websocket proxy to reduce open websockets at runtime

[niloc132]

Browsers only support a limited number of websockets at a time (limited by tab/origin/etc), and each grpc stream has a different purpose of some kind - autocomplete, checking for table size changes, and table data subscriptions. The actual value of the limit usually doesn't matter for a simple browser UI, but with PartitionedTable and plot-by implemented, it is necessary to support more concurrent streams than we previously have needed.

The simplest way to support this would be to transmit the "stream id" as part of each frame, so that each end of the connection knows what it is talking about.

There is an issue in improbable-eng/grpc-web that discusses this, and a branch with some in-progress work, but the solution there is to wrap with a new protobuf type. I think the simpler approach would be to alter the framing slightly, appending a 4byte stream id ahead of the proper grpc frame, similarly to how the existing websocket proxy uses a "control flow" byte to signal a half-close.

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Negotiation between client and server to support this multiplexing feature appears to be possible using websocket subprotocols, where a client that supports more than one system could ask the server to pick the "most specific" protocol it can speak. The downside of using this approach would be that each service has to connect directly, rather than letting multiple services share a single socket. This might not matter for us, as we don't have too many services, but it is something to think about.

Supporting multiple protocols lets us remain compatible with the upstream go proxy and "gracefully" fall back, but with the downside that using the go proxy will go back to the limited stream count behavior.