Server sent events (#1920)

## Issue Addressed

Resolves #1434 (this is the last major feature in the standard spec. There are only a couple of places we may be off-spec due to recent spec changes or ongoing discussion)
Partly addresses #1669
 
## Proposed Changes

- remove the websocket server
- remove the `TeeEventHandler` and `NullEventHandler` 
- add server sent events according to the eth2 API spec

## Additional Info

This is according to the currently unmerged PR here: https://github.com/ethereum/eth2.0-APIs/pull/117


Co-authored-by: realbigsean <seananderson33@gmail.com>

beacon_node/websocket_server/Cargo.toml

@@ -1,17 +0,0 @@
-[package]
-name = "websocket_server"
-version = "0.2.0"
-authors = ["Paul Hauner <paul@paulhauner.com>"]
-edition = "2018"
-
-# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
-
-[dependencies]
-futures = "0.3.7"
-serde = "1.0.116"
-serde_derive = "1.0.116"
-slog = "2.5.2"
-tokio = { version = "0.3.2", features = ["full"] }
-types = { path = "../../consensus/types" }
-ws = "0.9.1"
-task_executor = { path = "../../common/task_executor" }

beacon_node/websocket_server/src/config.rs

@@ -1,22 +0,0 @@
-use serde_derive::{Deserialize, Serialize};
-use std::net::Ipv4Addr;
-
-/// The core configuration of a Lighthouse beacon node.
-#[derive(Debug, Clone, Serialize, Deserialize)]
-pub struct Config {
-    pub enabled: bool,
-    /// The IPv4 address the REST API HTTP server will listen on.
-    pub listen_address: Ipv4Addr,
-    /// The port the REST API HTTP server will listen on.
-    pub port: u16,
-}
-
-impl Default for Config {
-    fn default() -> Self {
-        Config {
-            enabled: false,
-            listen_address: Ipv4Addr::new(127, 0, 0, 1),
-            port: 5053,
-        }
-    }
-}

beacon_node/websocket_server/src/lib.rs

@@ -1,118 +0,0 @@
-use slog::{debug, error, info, warn};
-use std::marker::PhantomData;
-use std::net::SocketAddr;
-use types::EthSpec;
-use ws::{Sender, WebSocket};
-
-mod config;
-
-pub use config::Config;
-
-pub struct WebSocketSender<T: EthSpec> {
-    sender: Option<Sender>,
-    _phantom: PhantomData<T>,
-}
-
-impl<T: EthSpec> WebSocketSender<T> {
-    /// Creates a dummy websocket server that never starts and where all future calls are no-ops.
-    pub fn dummy() -> Self {
-        Self {
-            sender: None,
-            _phantom: PhantomData,
-        }
-    }
-
-    pub fn send_string(&self, string: String) -> Result<(), String> {
-        if let Some(sender) = &self.sender {
-            sender
-                .send(string)
-                .map_err(|e| format!("Unable to broadcast to websocket clients: {:?}", e))
-        } else {
-            Ok(())
-        }
-    }
-}
-
-pub fn start_server<T: EthSpec>(
-    executor: task_executor::TaskExecutor,
-    config: &Config,
-) -> Result<(WebSocketSender<T>, SocketAddr), String> {
-    let log = executor.log();
-    let server_string = format!("{}:{}", config.listen_address, config.port);
-
-    // Create a server that simply ignores any incoming messages.
-    let server = WebSocket::new(|_| |_| Ok(()))
-        .map_err(|e| format!("Failed to initialize websocket server: {:?}", e))?
-        .bind(server_string.clone())
-        .map_err(|e| {
-            format!(
-                "Failed to bind websocket server to {}: {:?}",
-                server_string, e
-            )
-        })?;
-
-    let actual_listen_addr = server.local_addr().map_err(|e| {
-        format!(
-            "Failed to read listening addr from websocket server: {:?}",
-            e
-        )
-    })?;
-
-    let broadcaster = server.broadcaster();
-
-    // Produce a signal/channel that can gracefully shutdown the websocket server.
-    let exit = executor.exit();
-    let log_inner = log.clone();
-    let broadcaster_inner = server.broadcaster();
-    let exit_future = async move {
-        let _ = exit.await;
-        if let Err(e) = broadcaster_inner.shutdown() {
-            warn!(
-                log_inner,
-                "Websocket server errored on shutdown";
-                "error" => format!("{:?}", e)
-            );
-        } else {
-            info!(log_inner, "Websocket server shutdown");
-        }
-    };
-
-    // Place a future on the handle that will shutdown the websocket server when the
-    // application exits.
-
-    executor.spawn(exit_future, "Websocket exit");
-
-    let log_inner = log.clone();
-    let server_future = move || match server.run() {
-        Ok(_) => {
-            debug!(
-                log_inner,
-                "Websocket server thread stopped";
-            );
-        }
-        Err(e) => {
-            error!(
-                log_inner,
-                "Websocket server failed to start";
-                "error" => format!("{:?}", e)
-            );
-        }
-    };
-
-    executor.spawn_blocking(server_future, "Websocket server");
-
-    info!(
-        log,
-        "WebSocket server started";
-        "address" => format!("{}", actual_listen_addr.ip()),
-        "port" => actual_listen_addr.port(),
-    );
-
-    Ok((
-        WebSocketSender {
-            sender: Some(broadcaster),
-            _phantom: PhantomData,
-        },
-        actual_listen_addr,
-    ))
-}