AsyncResolve cleanups and error handling

The Async DBus resolver really has nothing to do with crow, which is our
core http library namespace and has some opportunistic cleanups that can
be done.

This commit moves it into the bmcweb namespace (unimportantly) and
breaks out one of the larger functions such that it can be unit tested,
and unit tests it.

Tested: Unit tests pass.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ie3cfbb0ef81a027a1ad42358c04967a517471117
diff --git a/http/http_client.hpp b/http/http_client.hpp
index 1533875..07fa85d 100644
--- a/http/http_client.hpp
+++ b/http/http_client.hpp
@@ -148,7 +148,7 @@
     std::function<void(bool, uint32_t, Response&)> callback;
 
 #ifdef BMCWEB_DBUS_DNS_RESOLVER
-    using Resolver = crow::async_resolve::Resolver;
+    using Resolver = async_resolve::Resolver;
 #else
     using Resolver = boost::asio::ip::tcp::resolver;
 #endif