feat: Make nixops4-eval async

A step toward handling the arrival of new data (stdin) with priority
over commands, avoiding roundtrips and re-evaluations.

(cherry picked from commit 8a2a5197886025caf35653001f76a4b209d8c9e4)

rust/nix-store/src/store.rs

@@ -32,6 +32,7 @@ impl Drop for StoreRef {
         }
     }
 }
+unsafe impl Send for StoreRef {}
 
 /// A [Weak] reference to a store.
 pub struct StoreWeak {