- Replaced most instances of PublicKey with KeyPair, since they need to be passed into each validator thread now.
- Pulled out a bunch of FreeAttestations, and replaced with regular Attestations (as per Paul's suggestion)
- Started generalising pubkeys to 'signers' (though they are still just Keypairs)
- Added validator_index into a few structs where relevant
- Removed the SlotClock and DutiesReader from the BlockProducer and Attester services, since this logic is now abstracted to the higher level process.
- Added a Hash trait to the Keypair (rather than just pubkey) which assumes the Pubkey uniquely defines it.
- Added a 'beacon_attester' RPC endpoint, so the BeaconNode can supply attestation data.
- Renamed 'attestation_data' to just 'attestation' throughout (except where it is actually just the data structure).
