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/cli.rs

@@ -232,29 +232,6 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                     address of this server (e.g., http://localhost:5054).")
                 .takes_value(true),
         )
-        /* Websocket related arguments */
-        .arg(
-            Arg::with_name("ws")
-                .long("ws")
-                .help("Enable the websocket server. Disabled by default.")
-                .takes_value(false),
-        )
-        .arg(
-            Arg::with_name("ws-address")
-                .long("ws-address")
-                .value_name("ADDRESS")
-                .help("Set the listen address for the websocket server.")
-                .default_value("127.0.0.1")
-                .takes_value(true),
-        )
-        .arg(
-            Arg::with_name("ws-port")
-                .long("ws-port")
-                .value_name("PORT")
-                .help("Set the listen TCP port for the websocket server.")
-                .default_value("5053")
-                .takes_value(true),
-        )
 
         /*
          * Standard staking flags