perf(storage): remove protobuf's copy of data on unmarshalling

[BrennaEpp]

• This reduces CPU usage and improves performance when running concurrent reads, since it avoids an extra copy of the data at the protobuf layer since it will unmarshal the data (which is already in a contiguous buffer in the message we receive) into another buffer. We do not need both buffers, so we can return the appropriate address in the buffer rather than copying it into another buffer to return. This is not supported by the proto library today, so we have to manually decode the message buffer.
• Tested performance and cpu usage in VM for large (100MB) and medium file (2MB) reads. The proto unmarshalling portion of cpu usage is no longer sampled (does not show up in the flame graphs).
• Passes unit and integration tests; still needs to be tested for retries

Note that this could be made even more efficient by parsing the whole block at once instead of reusing the readProtoBytes function, which will look at and then skip over the fields that have already been parsed. readProtoBytes calls protowire functions that do some calculations that re-slice the data buffer, without any copies. So, I don't think the improvement would be noticeable, especially since we only have 4 fields in the message. The cpu flame graphs do not show this function, so whatever time is spent there seems to be negligible.

cc: @dfawley

[tritone]

Overall looks good; just a few minor suggestions and questions

[tritone]

Looks good, nice work!

[neild]

Might want to note that this function is essentially identical to proto.Unmarshal, except it aliases the data in the input []byte. If we ever add a feature to Unmarshal that does that, this function can be dropped.

[neild]

You're unmarshaling all the fields in the message, so this will be a bit more efficient if you loop once over the fields rather than looking up each individually:

off := 0
for off < len(b) {
  num, typ, n := protowire.ConsumeTag(b[off:])
  if n < 0 {
    return nil, protowire.ParseError(n)
  }
  off += n
  switch {
  case num == checksummedDataField && typ == protowire.BytesType:
    // unmarshal the checksummed_data field
  case num == objectChecksumsField && typ == protowire.BytesType:
    // unmarshal the object_checksums field
  case num == contentRangeField && typ == protowire.BytesType:
    // unmarshal the content_range field
  case num == metadataField && typ == protowire.BytesType:
    // unmarshal the metadata field
  default:
    n = protowire.ConsumeFieldValue(num, typ, b[off:])
    if n < 0 {
      return nil, protowire.ParseError(n)
    }
    off += n
  }
}

[BrennaEpp]

Thanks for the suggestion!