Upgrade discovery and restructure task execution (#1693)

* Initial rebase

* Remove old code

* Correct release tests

* Rebase commit

* Remove eth2-testnet dep on eth2libp2p

* Remove crates lost in rebase

* Remove unused dep

beacon_node/network/src/beacon_processor/mod.rs

@@ -37,12 +37,12 @@
 
 use crate::{metrics, service::NetworkMessage, sync::SyncMessage};
 use beacon_chain::{BeaconChain, BeaconChainTypes, BlockError};
-use environment::TaskExecutor;
 use eth2_libp2p::{MessageId, NetworkGlobals, PeerId};
 use slog::{crit, debug, error, trace, warn, Logger};
 use std::collections::VecDeque;
 use std::sync::{Arc, Weak};
 use std::time::{Duration, Instant};
+use task_executor::TaskExecutor;
 use tokio::sync::{mpsc, oneshot};
 use types::{
     Attestation, AttesterSlashing, EthSpec, Hash256, ProposerSlashing, SignedAggregateAndProof,