Fix: Remove Sync requirements on Futures returned in the Rust plugin library.

See: https://github.com/bitcoindevkit/bdk/issues/1047#issuecomment-1660645669

In general, futures produced by most libraries in the ecosystem of Rust, and bounds placed
on users of famous runtimes like tokio and its spawn method all lack Sync requirements.

Because of this, anyone who creates a callback using any sort of library that returns a
non-Sync future (which most libraries fit this description) inside of it will get some
cryptic error messages (async error messages still leave a lot to be desired).

Removing these Sync requirements will make the library more useful.

plugins/src/lib.rs

@@ -104,7 +104,7 @@ where
 impl<S, I, O> Builder<S, I, O>
 where
     O: Send + AsyncWrite + Unpin + 'static,
-    S: Clone + Sync + Send + Clone + 'static,
+    S: Clone + Sync + Send + 'static,
     I: AsyncRead + Send + Unpin + 'static,
 {
     pub fn new(input: I, output: O) -> Self {
@@ -146,7 +146,7 @@ where
     where
         C: Send + Sync + 'static,
         C: Fn(Plugin<S>, Request) -> F + 'static,
-        F: Future<Output = Result<(), Error>> + Send + Sync + 'static,
+        F: Future<Output = Result<(), Error>> + Send + 'static,
     {
         self.subscriptions.insert(
             topic.to_string(),
@@ -162,7 +162,7 @@ where
     where
         C: Send + Sync + 'static,
         C: Fn(Plugin<S>, Request) -> F + 'static,
-        F: Future<Output = Response> + Send + Sync + 'static,
+        F: Future<Output = Response> + Send + 'static,
     {
         self.hooks.insert(
             hookname.to_string(),
@@ -179,7 +179,7 @@ where
     where
         C: Send + Sync + 'static,
         C: Fn(Plugin<S>, Request) -> F + 'static,
-        F: Future<Output = Response> + Send + Sync + 'static,
+        F: Future<Output = Response> + Send + 'static,
     {
         self.rpcmethods.insert(
             name.to_string(),
@@ -504,7 +504,7 @@ where
 
 impl<S> PluginDriver<S>
 where
-    S: Send + Clone + Sync,
+    S: Send + Clone,
 {
     /// Run the plugin until we get a shutdown command.
     async fn run<I, O>(