Message validation leftovers

[nkryuchkov]

Leftovers:

• [ ] Investigate rejected messages
  • [ ] message is duplicated (round change)
  • [ ] duplicated proposal with different data
  • [ ] signer is not leader
• [x] Remove the commented-out code in network/p2p/test_utils.go (https://github.com/ssvlabs/ssv/pull/1393#discussion_r1650112555, https://github.com/ssvlabs/ssv/pull/1393#discussion_r1667675468)
• [ ] Replace treemap with an array (https://github.com/ssvlabs/ssv/pull/1393#discussion_r1664436261)
• [ ] Optimize constructing validator indices (https://github.com/ssvlabs/ssv/pull/1393#discussion_r1667681686)
• [ ] Check ValidatorRegistration duty (https://github.com/ssvlabs/ssv/pull/1393#discussion_r1667687976)
• [ ] Check all constants (https://github.com/ssvlabs/ssv/pull/1393#discussion_r1670160504)