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/network/src/attestation_service/tests/mod.rs

@@ -4,7 +4,6 @@ mod tests {
     use beacon_chain::{
         builder::{BeaconChainBuilder, Witness},
         eth1_chain::CachingEth1Backend,
-        events::NullEventHandler,
     };
     use futures::Stream;
     use genesis::{generate_deterministic_keypairs, interop_genesis_state};
@@ -25,7 +24,6 @@ mod tests {
         SystemTimeSlotClock,
         CachingEth1Backend<MinimalEthSpec>,
         MinimalEthSpec,
-        NullEventHandler<MinimalEthSpec>,
         MemoryStore<MinimalEthSpec>,
         MemoryStore<MinimalEthSpec>,
     >;
@@ -61,7 +59,6 @@ mod tests {
                     .expect("should build state using recent genesis")
                     .dummy_eth1_backend()
                     .expect("should build dummy backend")
-                    .null_event_handler()
                     .slot_clock(SystemTimeSlotClock::new(
                         Slot::new(0),
                         Duration::from_secs(recent_genesis_time()),

beacon_node/network/src/beacon_processor/worker/gossip_methods.rs

@@ -1,4 +1,5 @@
 use crate::{metrics, service::NetworkMessage, sync::SyncMessage};
+
 use beacon_chain::{
     attestation_verification::Error as AttnError, observed_operations::ObservationOutcome,
     BeaconChainError, BeaconChainTypes, BlockError, ForkChoiceError,
@@ -371,6 +372,7 @@ impl<T: BeaconChainTypes> Worker<T> {
         self.propagate_validation_result(message_id, peer_id, MessageAcceptance::Accept);
 
         self.chain.import_voluntary_exit(exit);
+
         debug!(self.log, "Successfully imported voluntary exit");
 
         metrics::inc_counter(&metrics::BEACON_PROCESSOR_EXIT_IMPORTED_TOTAL);