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/rpc/src/beacon_attester.rs

@@ -0,0 +1,61 @@
+use futures::Future;
+use grpcio::{RpcContext, UnarySink};
+use protos::services::{
+    Attestation as AttestationProto, ProduceAttestation, ProduceAttestationResponse,
+    ProduceAttestationRequest, PublishAttestationResponse, PublishAttestationRequest,
+    PublishAttestation
+};
+use protos::services_grpc::BeaconBlockService;
+use slog::Logger;
+
+#[derive(Clone)]
+pub struct AttestationServiceInstance {
+    pub log: Logger,
+}
+
+impl AttestationService for AttestationServiceInstance {
+    /// Produce a `BeaconBlock` for signing by a validator.
+    fn produce_attestation(
+        &mut self,
+        ctx: RpcContext,
+        req: ProduceAttestationRequest,
+        sink: UnarySink<ProduceAttestationResponse>,
+    ) {
+        println!("producing attestation at slot {}", req.get_slot());
+
+        // TODO: build a legit block.
+        let mut attestation = AttestationProto::new();
+        attestation.set_slot(req.get_slot());
+        // TODO Set the shard to something legit.
+        attestation.set_shard(0);
+        attestation.set_block_root(b"cats".to_vec());
+
+        let mut resp = ProduceAttestationResponse::new();
+        resp.set_attestation_data(attestation);
+
+        let f = sink
+            .success(resp)
+            .map_err(move |e| println!("failed to reply {:?}: {:?}", req, e));
+        ctx.spawn(f)
+    }
+
+    /// Accept some fully-formed `BeaconBlock`, process and publish it.
+    fn publish_attestation(
+        &mut self,
+        ctx: RpcContext,
+        req: PublishAttestationRequest,
+        sink: UnarySink<PublishAttestationResponse>,
+    ) {
+        println!("publishing attestation {:?}", req.get_block());
+
+        // TODO: actually process the block.
+        let mut resp = PublishAttestationResponse::new();
+
+        resp.set_success(true);
+
+        let f = sink
+            .success(resp)
+            .map_err(move |e| println!("failed to reply {:?}: {:?}", req, e));
+        ctx.spawn(f)
+    }
+}