Remove Future Sync bound from ServiceBuilder::boxed_clone_sync() (#851)

The function Future bounds are slightly too restrictive. Namely:
```
<L::Service as Service<R>>::Future: Send + Sync + 'static,
```

The `BoxCloneSyncService` and `BoxCloneSyncServiceLayer` require:
```
S::Future: Send + 'static,
<L::Service as Service<T>>::Future: Send + 'static,
```

Removing the `Sync` bound from the helper function:
 - Makes the helper function more useful (where futures are not `Sync`)
 - Is backwards compatible (loosening trait bound)

Author: garypen

tower/CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+# Unreleased
+
+### Changed
+
+- **builder**: Remove Future Sync bound from ServiceBuilder::boxed_clone_sync() ([#851])
+
 # 0.5.3
 
 ### Added

tower/src/builder/mod.rs

@@ -845,7 +845,7 @@ impl<L> ServiceBuilder<L> {
     where
         L: Layer<S> + Send + Sync + 'static,
         L::Service: Service<R> + Clone + Send + Sync + 'static,
-        <L::Service as Service<R>>::Future: Send + Sync + 'static,
+        <L::Service as Service<R>>::Future: Send + 'static,
     {
         let layer = self.into_inner();