Fix newly reported clippy errors

- Use variables directly in format strings.
  As reported by clippy, variables can be used directly in the
  `format!` string.
- Use rewind() instead of seeking to 0.
- Remove superfluous & and ref.

Signed-off-by: Petr Tesarik <petr@tesarici.cz>

core/src/mercury/mod.rs

@@ -263,7 +263,7 @@ impl MercuryManager {
             let mut found = false;
 
             self.lock(|inner| {
-                inner.subscriptions.retain(|&(ref prefix, ref sub)| {
+                inner.subscriptions.retain(|(prefix, sub)| {
                     if encoded_uri.starts_with(prefix) {
                         found = true;