Reduce QBFT throughput

[corverroos]

Problem to be solved

QBFT currently uses a lot of bandwidth (network bytes sent and received). So much so, that memory spikes to 1GB when doing block proposals of 300KB. This is due to a lot of duplication in the protocol and wire formats.

Proposed solution

Incorporate these optimisations:

• use SSZ compression for proposed values (instead of json), saves 50%.
• Use nested messages with signature wrapper types to avoid copying protos when verifying signatures, saves 50%.
• QBFT messages should only contain hashes, only include uniq map of hashes to original values in a wrapper type, saves 90%.
• Avoid all in-memory duplication of original values. Only buffer hash based QBFT messages.

This is a big protocol level refactor, implement this as QBFT v2 and use prioritise to enable/disable.