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/src/config.rs

@@ -150,26 +150,6 @@ pub fn get_config<E: EthSpec>(
         );
     }
 
-    /*
-     * Websocket server
-     */
-
-    if cli_args.is_present("ws") {
-        client_config.websocket_server.enabled = true;
-    }
-
-    if let Some(address) = cli_args.value_of("ws-address") {
-        client_config.websocket_server.listen_address = address
-            .parse::<Ipv4Addr>()
-            .map_err(|_| "ws-address is not a valid IPv4 address.")?;
-    }
-
-    if let Some(port) = cli_args.value_of("ws-port") {
-        client_config.websocket_server.port = port
-            .parse::<u16>()
-            .map_err(|_| "ws-port is not a valid u16.")?;
-    }
-
     /*
      * Eth1
      */
@@ -250,7 +230,6 @@ pub fn get_config<E: EthSpec>(
             unused_port("udp").map_err(|e| format!("Failed to get port for discovery: {}", e))?;
         client_config.http_api.listen_port = 0;
         client_config.http_metrics.listen_port = 0;
-        client_config.websocket_server.port = 0;
     }
 
     /*