transfer: do not use EXPIRE_NOW while blocked

- When a transfer sets `data->state.select_bits`, it is
  scheduled for rerun with EXPIRE_NOW. If such a transfer
  is blocked (due to PAUSE, for example), this will lead to
  a busy loop.
- multi.c: check for transfer block
- sendf.*: add Curl_xfer_is_blocked()
- sendf.*: add client reader `is_paused()` callback
- implement is_paused()` callback where needed

Closes #13908

lib/c-hyper.c

@@ -1206,6 +1206,7 @@ static const struct Curl_crtype cr_hyper_protocol = {
   Curl_creader_def_resume_from,
   Curl_creader_def_rewind,
   cr_hyper_unpause,
+  Curl_creader_def_is_paused,
   Curl_creader_def_done,
   sizeof(struct Curl_creader)
 };