Fix bug that pause/unpause seeding torrent prevents new downloads from starting

Previously, with --bt-detach-seed-only, when user issued pause and
unpause command to seeding torrent, and it was started again, and it
immediately got seeding status, but aria2 did not start new download
waiting in the queue.  This commit fixes this bug.

See GH-490

src/RequestGroup.cc

@@ -984,6 +984,9 @@ void RequestGroup::releaseRuntimeResource(DownloadEngine* e)
   // progress information via RPC
   progressInfoFile_ = std::make_shared<NullProgressInfoFile>();
   downloadContext_->releaseRuntimeResource();
+  // Reset seedOnly_, so that we can handle pause/unpause-ing seeding
+  // torrent with --bt-detach-seed-only.
+  seedOnly_ = false;
 }
 
 void RequestGroup::preDownloadProcessing()