Add validator_node, restructure binaries, gRPC.

This is a massive commit which restructures the workspace, adds a very
basic, untested, validator client and some very basic, non-functioning
gRPC endpoints to the beacon-node.

beacon_node/db/src/traits.rs

@@ -0,0 +1,28 @@
+pub type DBValue = Vec<u8>;
+
+#[derive(Debug)]
+pub struct DBError {
+    pub message: String,
+}
+
+impl DBError {
+    pub fn new(message: String) -> Self {
+        Self { message }
+    }
+}
+
+/// A generic database to be used by the "client' (i.e.,
+/// the lighthouse blockchain client).
+///
+/// The purpose of having this generic trait is to allow the
+/// program to use a persistent on-disk database during production,
+/// but use a transient database during tests.
+pub trait ClientDB: Sync + Send {
+    fn get(&self, col: &str, key: &[u8]) -> Result<Option<DBValue>, DBError>;
+
+    fn put(&self, col: &str, key: &[u8], val: &[u8]) -> Result<(), DBError>;
+
+    fn exists(&self, col: &str, key: &[u8]) -> Result<bool, DBError>;
+
+    fn delete(&self, col: &str, key: &[u8]) -> Result<(), DBError>;
+}