Align to v0.12.3

[terencechain]

Opening this to track v0.12.3 changes. This is 100% backwards compatible with v0.12.x

Beacon chain

• Add configuration -- PROPORTIONAL_SLASHING_MULTIPLIER -- for slashing multiplier (Add a constant for slash penalty multiplier ethereum/consensus-specs#2034)
• [non-substantive] Add clarifying comment for genesis skip conditions (add clarifying comment for genesis skip conditions ethereum/consensus-specs#1999)

Fork choice

• [non-substantive] Pass anchor_block to get_forkchoice_store (Pass anchor_block to get_forkchoice_store ethereum/consensus-specs#2061)

Validator

• [non-substantive] Clarify what state is for block proposals in validator guide (clarify what 'state' is for block proposals in validator guide ethereum/consensus-specs#2022)

Networking

• Add self-consistency checks to attestation gossip validation (Add self-consistency check to attestation gossip validation ethereum/consensus-specs#2001)
• Add two more simple gossip validations (Two more gossip validations ethereum/consensus-specs#2030)
• Use raw SHA256 as message-id and reduce size to 8 bytes (use raw SHA256 as message-id and reduce size to 8 bytes ethereum/consensus-specs#2044)

Simple Serialize

• [non-substantive] Clarify offsets serialization (clarify offsets serialization ethereum/consensus-specs#2043)

BLS

• [non-substantive] Bump IETF BLS standard version to -03 (Bump IETF BLS standard version to -03 ethereum/consensus-specs#2051)

I'm neglecting the wss changes since we already are tracking that separately