[examples/eth-dm] Do not use JSON over the wire

[D4nte]

Problem

JSON is not data efficient, see https://forum.vac.dev/t/eth-dm-learnings-and-what-is-next/84/2

Solution

Use compressed JSON instead. See #214 (comment)
Use protobuf.

[D4nte]

There does not see to be a widely adopted standard for JSON compression.

There are a number of library that attempt to compress the JSON format to reduce data duplication:

• https://www.npmjs.com/package/compress-json
• https://www.npmjs.com/package/compressed-json

Others propose to use a different format coupled with a generic compression algorithm:

• https://www.lucidchart.com/techblog/2019/12/06/json-compression-alternative-binary-formats-and-compression-methods/
• https://coderwall.com/p/mekopw/jsonc-compress-your-json-data-up-to-80

Because there is no standard I can refer too, I do not think it would be a good idea to used compressed JSON.

However, the fact that JSON is a not a size efficient format, it would still be good to replace it with something else. Alternatives are:

• protobuf: as done with other protocols
• msgpack: https://msgpack.org/ (js implementation: https://www.npmjs.com/package/@msgpack/msgpack)

In this occurrence, it may be best to just role with protobuf. We can review at a later stage if there are other format worth introducing.