Update dashmap to v5.1.0

This reinstates the v5 version of `dashmap`, which previously had to be
reverted due to a soundness issue that has since been fixed in the
v5.1.0 version.

Cargo.toml

@@ -146,7 +146,7 @@ version = "0.3.16"
 version = "2.0"
 
 [dependencies.dashmap]
-version = "4.0.2"
+version = "5.1.0"
 features = ["serde"]
 optional = true
 

src/cache/mod.rs

@@ -95,9 +95,7 @@ impl<F: FromStr> FromStrAndCache for F {
 
 /// Iterator given to the selector closure in [`Cache::channel_messages_field`].
 // Wrapper around a specific iterator type to allow swapping out iterators on cache design changes
-//
-// Clone impl waiting on this https://github.com/xacrimon/dashmap/pull/152
-//#[derive(Clone)]
+#[derive(Clone)]
 pub struct MessageIterator<'a, S: BuildHasher + Clone>(
     Iter<'a, MessageId, Message, S, DashMap<MessageId, Message, S>>,
 );