Opt-in Slashing protection + interchange

[mratsim]

This adds slashing protection to the Beacon Node and validator client

State

• Slashing protection engine
  • Slashing protection for duplicate block proposal slots
  • Slashing protection for duplicate attestation target epochs
  • Slashing protection for surrounding votes
  • Slashing protection for surrounded votes
  • Optional DB pruning (incompatible with "complete" export)
• Integration in Beacon Node and Validator Client
  • Block proposal
  • Attestation
• Interchange
  • "Complete" format export
  • "minimal" format export
  • "Complete" format import
  • "minimal" format import
• Optimization
  • Avoid duplicate compute_block_root / compute_attestation_root

Overview

The slashing protection is introduced as an experimental feature that can be opt-in with
make NIMFLAGS="-d:UseSlashingProtection=true" beacon_node or make NIMFLAGS="-d:UseSlashingProtection=true" validator_client
For now only block proposal are protected, protecting attestation requires to add registerAttestation and notSlashableAttestation in their proper place in validator_duties.nim and validator_client.nim

Interchange

Complete export can be done by calling "toSPDIF" on the slashing protection database, likely to be added to ncli_db.

Design

The slashing protection uses an independent database that is attached to either the Beacon Node or the Validator Client. It's a KV-Store but a classic SQL DB would significantly reduce the potential bug surface as right now I'm using a linked list in the DB field which accounts for a lot of lines of code. Alternatively, better support for range queries for the KV-Store would be nice

[mratsim]

Not draft anymore.

I've added an extra temporary test with/without slashing protection for jenkins

• make sure that without slashing protection, we don't access the DB and so no null pointer exception
• make sure that we finalized with slashing protection instead of being stuck in a endless loop
  Once that succeed we'll only keep slashing protection in Jenkins (as tests are long)

Regarding import/export AFAIK no one actually supports them yet in production code (but does in PR) so no real rush but importing a complete DB is straightforward. Adding that in this PR.