switch Iterator to Send + Iterator in return types

This changes the SQLite implementation quite a bit. It may potentially
reduce performance, but allows using the new async API and removes some
usage of unsafe

src/service/pusher/mod.rs

@@ -39,7 +39,7 @@ impl Service {
         self.db.get_pushers(sender)
     }
 
-    pub fn get_pushkeys(&self, sender: &UserId) -> Box<dyn Iterator<Item = Result<String>>> {
+    pub fn get_pushkeys(&self, sender: &UserId) -> Box<dyn Send + Iterator<Item = Result<String>>> {
         self.db.get_pushkeys(sender)
     }