Verify attestations in blocks

[mkalinin]

Abstract

Verify block's attestation records on before the import.

Proposed implementation

Implement a couple of new ValidationRule and add it to the BeaconValidator.
Rules should do following checks:

• Attestation record with 0 index contains distinct valid attestation that is created by proposer of that block
• Attestation records with indices > 0 are valid records and were created by attesters from committee that is eligible to attest on block's slot number

Consider block as invalid if any of these two checks fails.

[zilm13]

resolved with #1219