Progress towards validator signing attestations.

- 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).

beacon_node/beacon_chain/test_harness/src/validator_harness/direct_beacon_node.rs

@@ -50,18 +50,18 @@ impl<T: ClientDB, U: SlotClock, F: ForkChoice> DirectBeaconNode<T, U, F> {
 }
 
 impl<T: ClientDB, U: SlotClock, F: ForkChoice> AttesterBeaconNode for DirectBeaconNode<T, U, F> {
-    fn produce_attestation_data(
+    fn produce_attestation(
         &self,
         _slot: Slot,
         shard: u64,
     ) -> Result<Option<AttestationData>, NodeError> {
-        match self.beacon_chain.produce_attestation_data(shard) {
+        match self.beacon_chain.produce_attestation(shard) {
             Ok(attestation_data) => Ok(Some(attestation_data)),
             Err(e) => Err(NodeError::RemoteFailure(format!("{:?}", e))),
         }
     }
 
-    fn publish_attestation_data(
+    fn publish_attestation(
         &self,
         free_attestation: FreeAttestation,
     ) -> Result<AttestationPublishOutcome, NodeError> {