Trigger thread dump before AsyncHttpClientSpec times out

Mix new `ThreadDumpOnTimeout` into `AsyncHttpClientSpec` and print
thread dump to console 500ms before the spec times out.  Hoping to
better understand:
https://github.com/http4s/http4s/issues/858http4s/http4s#issuecomment-274133582
and also apply `ThreadDumpOnTimeout` to understand http4s/http4s#774.

async-http-client/src/test/scala/org/http4s/client/asynchttpclient/AsyncHttpClientSpec.scala

@@ -1,5 +1,10 @@
 package org.http4s.client.asynchttpclient
 
 import org.http4s.client.ClientRouteTestBattery
+import org.http4s.testing.ThreadDumpOnTimeout
 
-class AsyncHttpClientSpec extends ClientRouteTestBattery("AsyncHttpClient", AsyncHttpClient())
+import scala.concurrent.duration._
+
+class AsyncHttpClientSpec extends ClientRouteTestBattery("AsyncHttpClient", AsyncHttpClient()) with ThreadDumpOnTimeout {
+  override val triggerThreadDumpAfter = timeout - 500.millis
+}