misc changes

* implement socket timeout error
* use 128k buffer size for copy
* use NewPipe HTTP user agent in the downloads
* automatically recover downloads with network errors that are queued

app/src/main/java/us/shandian/giga/get/DownloadRunnable.java

@@ -27,7 +27,6 @@ public class DownloadRunnable extends Thread {
         if (mission == null) throw new NullPointerException("mission is null");
         mMission = mission;
         mId = id;
-        mConn = null;
     }
 
     @Override