replace instant with web-time (#5800)

* replace instant with web_time

* Merge branch 'unstable' into replace-instant-gossipsub

# Conflicts:
#	Cargo.lock
#	beacon_node/lighthouse_network/Cargo.toml

beacon_node/lighthouse_network/gossipsub/src/time_cache.rs

@@ -21,13 +21,13 @@
 //! This implements a time-based LRU cache for checking gossipsub message duplicates.
 
 use fnv::FnvHashMap;
-use instant::Instant;
 use std::collections::hash_map::{
     self,
     Entry::{Occupied, Vacant},
 };
 use std::collections::VecDeque;
 use std::time::Duration;
+use web_time::Instant;
 
 struct ExpiringElement<Element> {
     /// The element that expires