[meshnet] Use gRPC streaming

[kraney]

Note: Stacks on top of #731, new changes start at sha:6c4cadb3e02b85a880015fa7561ae40d2ad12d53. Please merge that PR first.

  1. Bidirectional Streaming (SendToStream) Receiver (handler.go):
      • Implemented SendToStream(stream mpb.WireProtocol_SendToStreamServer) error.
      • In a loop, stream.Recv() continuously ingests incoming mpb.Packet frames and writes them directly to the
      destination TAP interface (wrHandle.Write(pkt.Frame)), bypassing per-packet unary RPC overhead.
  2. Streaming Sender & Auto-Reconnect (grpcwire.go):
      • Updated RecvFrmLocalPodThread to establish a persistent client stream (wireClient.SendToStream(ctx)).
      • Frames read from the local TAP interface are streamed out via st.Send(payload) without blocking for individual
      RPC responses.
      • If the stream encounters a network or peer reset, RecvFrmLocalPodThread automatically clears its stream handle
      and transparently re-establishes SendToStream on the next packet.
  3. HTTP/2 Window Size & Buffer Tuning:
      • Server Configuration (meshnet.go):
          • Stream window size: 4 MB (grpc.InitialWindowSize(4 * 1024 * 1024)).
          • Connection window size: 16 MB (grpc.InitialConnWindowSize(16 * 1024 * 1024)).
          • Max message payload: 64 MB (grpc.MaxRecvMsgSize / grpc.MaxSendMsgSize).
      • Client Configuration (grpcwire.go):
          • Configured matching initial window size and message payload limits on client grpc.Dial.