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/config.rs

@@ -59,7 +59,6 @@ pub struct Config {
     pub store: store::StoreConfig,
     pub network: network::NetworkConfig,
     pub chain: beacon_chain::ChainConfig,
-    pub websocket_server: websocket_server::Config,
     pub eth1: eth1::Config,
     pub http_api: http_api::Config,
     pub http_metrics: http_metrics::Config,
@@ -77,7 +76,6 @@ impl Default for Config {
             store: <_>::default(),
             network: NetworkConfig::default(),
             chain: <_>::default(),
-            websocket_server: <_>::default(),
             dummy_eth1_backend: false,
             sync_eth1_chain: false,
             eth1: <_>::default(),