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/websocket_server/Cargo.toml

@@ -1,17 +0,0 @@
-[package]
-name = "websocket_server"
-version = "0.2.0"
-authors = ["Paul Hauner <paul@paulhauner.com>"]
-edition = "2018"
-
-# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
-
-[dependencies]
-futures = "0.3.7"
-serde = "1.0.116"
-serde_derive = "1.0.116"
-slog = "2.5.2"
-tokio = { version = "0.3.2", features = ["full"] }
-types = { path = "../../consensus/types" }
-ws = "0.9.1"
-task_executor = { path = "../../common/task_executor" }