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/client/src/lib.rs

@@ -27,7 +27,6 @@ pub struct Client<T: BeaconChainTypes> {
     http_api_listen_addr: Option<SocketAddr>,
     /// Listen address for the HTTP server which serves Prometheus metrics.
     http_metrics_listen_addr: Option<SocketAddr>,
-    websocket_listen_addr: Option<SocketAddr>,
 }
 
 impl<T: BeaconChainTypes> Client<T> {
@@ -46,11 +45,6 @@ impl<T: BeaconChainTypes> Client<T> {
         self.http_metrics_listen_addr
     }
 
-    /// Returns the address of the client's WebSocket API server, if it was started.
-    pub fn websocket_listen_addr(&self) -> Option<SocketAddr> {
-        self.websocket_listen_addr
-    }
-
     /// Returns the port of the client's libp2p stack, if it was started.
     pub fn libp2p_listen_port(&self) -> Option<u16> {
         self.network_globals.as_ref().map(|n| n.listen_port_tcp())