Merged Age's changes and ripped out heaps of now obsolete stuff in the validator client.

- 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.

eth2/attester/src/traits.rs

@@ -1,4 +1,4 @@
-use types::{AttestationData, FreeAttestation, Signature, Slot};
+use types::{AttestationData, Attestation, Signature, Slot};
 
 #[derive(Debug, PartialEq, Clone)]
 pub enum BeaconNodeError {
@@ -22,7 +22,7 @@ pub trait BeaconNode: Send + Sync {
 
     fn publish_attestation(
         &self,
-        free_attestation: FreeAttestation,
+        attestation: Attestation,
     ) -> Result<PublishOutcome, BeaconNodeError>;
 }