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>

common/eth2/Cargo.toml

@@ -11,7 +11,7 @@ serde = { version = "1.0.116", features = ["derive"] }
 serde_json = "1.0.58"
 types = { path = "../../consensus/types" }
 hex = "0.4.2"
-reqwest = { version = "0.10.8", features = ["json"] }
+reqwest = { version = "0.10.8", features = ["json","stream"] }
 eth2_libp2p = { path = "../../beacon_node/eth2_libp2p" }
 proto_array = { path = "../../consensus/proto_array", optional = true }
 serde_utils = { path = "../../consensus/serde_utils" }
@@ -23,6 +23,8 @@ bytes = "0.5.6"
 account_utils = { path = "../../common/account_utils" }
 eth2_ssz = "0.1.2"
 eth2_ssz_derive = "0.1.0"
+futures-util = "0.3.8"
+futures = "0.3.8"
 
 [target.'cfg(target_os = "linux")'.dependencies]
 psutil = { version = "3.2.0", optional = true }