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/beacon_chain/Cargo.toml

@@ -42,7 +42,6 @@ tree_hash = "0.1.1"
 types = { path = "../../consensus/types" }
 tokio = "0.3.2"
 eth1 = { path = "../eth1" }
-websocket_server = { path = "../websocket_server" }
 futures = "0.3.7"
 genesis = { path = "../genesis" }
 integer-sqrt = "0.1.5"
@@ -56,7 +55,6 @@ bls = { path = "../../crypto/bls" }
 safe_arith = { path = "../../consensus/safe_arith" }
 fork_choice = { path = "../../consensus/fork_choice" }
 task_executor = { path = "../../common/task_executor" }
-bus = "2.2.3"
 derivative = "2.1.1"
 itertools = "0.9.0"
 regex = "1.3.9"