bug: doesn't advertise compression

[blampe]

#41 hooked up zstd, but --remote_cache_compression=true against a clientd config like this fails:

contentAddressableStorage: {
  grpc: {
    client: { address: 'upstream-cluster:443', tls: {} },
    enableCompression: true,
  },
},

The bb-storage layer is correctly advertising compression, but bb-clientd doesn't forward the server's compression capability I think due to this.

[EdSchouten]

This is intentional. There is very little use in letting Bazel use compression when talking to bb_clientd. That only ends up wasting CPU cycles. UNIX sockets should be fast enough to transmit data in uncompressed form.