Validator client slashing protection

[arnetheduck]

When attesting, we need to be careful not to create conflicting attestations that can lead to the validator being slashed - here's a useful doc (hats off to sigp):

https://hackmd.io/u8MTIe5IRmybzVz38-sGdQ?view

Proof of concept implementation:
https://github.com/pscott/attestation_slashing_protection/

[mratsim]

See slashing detection write-up: https://hackmd.io/@n0ble/By897a5sH

References:

• Casper FFG paper https://arxiv.org/pdf/1710.09437.pdf
• Weak subjectivity evaluation https://ethresear.ch/t/weak-subjectivity-under-the-exit-queue-model/5187
• Sharding attestation policing sharding attestation policing across validators/nodes ethereum/eth2.0-pm#63
• eth2-surround by Diederik (@protolambda) https://github.com/protolambda/eth2-surround#min-max-surround

[mratsim]

Closed by #1757 #1643 #1804