[comm-lib] Add Send requirement for Defer closures

Summary:
This is to make `Defer` work in fully asynchronous context. It was working in actix-web without this because every HTTP listener is single-threaded there. For tonic gRPC, closures must be `Send` in RPC handlers in order to pass them between `await` calls

Depends on D14281

Test Plan: cargo check

Reviewers: kamil, tomek

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D14282

shared/comm-lib/src/tools.rs

@@ -46,13 +46,17 @@ pub type BoxedError = Box<dyn std::error::Error>;
 ///     }
 /// }
 /// ```
-pub struct Defer<'s>(Option<Box<dyn FnOnce() + 's>>);
+pub struct Defer<'s>(Option<Box<dyn FnOnce() + 's + Send>>);
 
 impl<'s> Defer<'s> {
-  pub fn new(f: impl FnOnce() + 's) -> Self {
+  pub fn new(f: impl FnOnce() + 's + Send) -> Self {
     Self(Some(Box::new(f)))
   }
 
+  pub fn empty() -> Self {
+    Self(None)
+  }
+
   /// Consumes the value, without calling the provided function
   ///
   /// # Example