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/connect/http.rs

@@ -77,9 +77,8 @@ impl HttpConnector {
     /// Construct a new HttpConnector.
     ///
     /// Takes number of DNS worker threads.
-    #[inline]
-    pub fn new(threads: usize) -> HttpConnector {
-        HttpConnector::new_with_resolver(GaiResolver::new(threads))
+    pub fn new() -> HttpConnector {
+        HttpConnector::new_with_resolver(GaiResolver::new())
     }
 }