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/utils/bls/src/keypair.rs

@@ -1,5 +1,6 @@
 use super::{PublicKey, SecretKey};
 use serde_derive::{Deserialize, Serialize};
+use std::hash::{Hash, Hasher};
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
 pub struct Keypair {
@@ -19,3 +20,15 @@ impl Keypair {
         self.pk.concatenated_hex_id()
     }
 }
+
+impl Hash for Keypair {
+    /// Note: this is distinct from consensus serialization, it will produce a different hash.
+    ///
+    /// This method uses the uncompressed bytes, which are much faster to obtain than the
+    /// compressed bytes required for consensus serialization.
+    ///
+    /// Use `ssz::Encode` to obtain the bytes required for consensus hashing.
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.pk.as_uncompressed_bytes().hash(state)
+    }
+}