ntf server: concurrent APNS push via sendRequestDirect (#1780)

The per-(srvHost, provider) worker shards added in #1779 still funnel
all APNS sends through one HTTP2Client's reqQ, where a single process
thread calls sendRequest serially - one in-flight HTTP/2 stream at a
time, capping APNS throughput at 1/RTT.

sendRequestDirect bypasses the queue and invokes sendReq directly from
the calling worker, so concurrent workers open parallel HTTP/2 streams
on the shared APNS connection and the multiplexing happens on the wire.

src/Simplex/Messaging/Notifications/Server/Push/APNS.hs

@@ -346,7 +346,7 @@ apnsPushProviderClient c@APNSPushClient {nonceDrg, apnsCfg} tkn@NtfTknRec {token
   nonce <- atomically $ C.randomCbNonce nonceDrg
   apnsNtf <- liftEither $ first PPCryptoError $ apnsNotification tkn nonce (paddedNtfLength apnsCfg) pn
   req <- liftIO $ apnsRequest c tknStr apnsNtf
-  HTTP2Response {response, respBody = HTTP2Body {bodyHead}} <- liftHTTPS2 $ sendRequest http2 req Nothing
+  HTTP2Response {response, respBody = HTTP2Body {bodyHead}} <- liftHTTPS2 $ sendRequestDirect http2 req Nothing
   let status = H.responseStatus response
       reason' = maybe "" reason $ J.decodeStrict' bodyHead
   if status == Just N.ok200