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>

common/warp_utils/src/reject.rs

@@ -1,7 +1,24 @@
 use eth2::types::{ErrorMessage, Failure, IndexedErrorMessage};
 use std::convert::Infallible;
+use std::error::Error;
+use std::fmt;
 use warp::{http::StatusCode, reject::Reject};
 
+#[derive(Debug)]
+pub struct ServerSentEventError(pub String);
+
+impl Error for ServerSentEventError {}
+
+impl fmt::Display for ServerSentEventError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{:?}", self)
+    }
+}
+
+pub fn server_sent_event_error(s: String) -> ServerSentEventError {
+    ServerSentEventError(s)
+}
+
 #[derive(Debug)]
 pub struct BeaconChainError(pub beacon_chain::BeaconChainError);