feat(client): change GaiResolver to use a global blocking threadpool

BREAKING CHANGE: Calls to `GaiResolver::new` and `HttpConnector::new` no
  longer should pass an integer argument for the number of threads.

src/client/mod.rs

@@ -1023,7 +1023,7 @@ impl Builder {
         B: Payload + Send,
         B::Data: Send,
     {
-        let mut connector = HttpConnector::new(4);
+        let mut connector = HttpConnector::new();
         if self.pool_config.enabled {
             connector.set_keepalive(self.pool_config.keep_alive_timeout);
         }