Changes from bitshares repo:

Author: Peter Conrad <cyrano@quisquis.de>
Date:   Thu May 2 16:25:23 2019 +0200

    Moved retain/release to task_base

commit 328ff59e938cbe3af844bd6474069d3608e25d03
Author: Peter Conrad <cyrano@quisquis.de>
Date:   Sat Apr 27 16:29:55 2019 +0200

    Added retain where applicable

commit 147baaea0df5c8b6f3d49395c196f7b99a6073ae
Author: Peter Conrad <cyrano@quisquis.de>
Date:   Fri Apr 26 14:21:11 2019 +0200

    Intermediate

commit 55ad023d9426ce48411cb3490eb9893107ef572e
Author: Dmitry Yakovitsky <d.yakovitsky@aetsoft.by>
Date:   Tue May 14 17:31:28 2019 +0300

    Fix memory leak. Not all tasks are deleted in thread_d dtor.