Optimize node serialization with stream vbyte

[yihuang]

Experiment: #673

Stream-vbyte¹ is a faster alternative than variant in protobuf. Good to try it to optimize node serialization. The experiment shows some good improvements.

Several points:

• we don't have enough integers to encode to reach full potential of SIMP, we have 4 int32(version/size-lo/size-hi/keyLen), assuming int32 is enough for version, is it profitable to batch multiple nodes together?
• the golang implementation seems fail to build in CI
• the scalar implementation seems simple enough
• changing node encoding format need full db migration
• hash computation use varint as well, but I guess changing that make it harder to migrate

Footnotes

1. https://github.com/lemire/streamvbyte ↩