Fixing an issue with asnyc http request handlers.

The current poltergeist http api immediately returns
and does not wait for async work in the handler to finish. This
mostly occurs when a public asap key needs to be fetched due
to a cache miss. The fix implements the strategy described at
https://prosody.im/doc/developers/http.html

resources/prosody-plugins/util.lib.lua

@@ -62,15 +62,17 @@ end
 
 
 function wrap_async_run(event,handler)
-    local result;
+    -- Grab a local response so that we can send the http response when
+    -- the handler is done.
+    local response = event.response;
     local async_func = runner(function (event)
-        local wait, done = waiter();
-        result=handler(event);
-        done();
-        return result;
+          response.status_code = handler(event);
+          -- Send the response to the waiting http client.
+          response:send();
     end)
     async_func:run(event)
-    return result;
+    -- return true to keep the client http connection open.
+    return true;
 end
 
 --- Updates presence stanza, by adding identity node