Kill worker when calling SequentialExecutor#shutdownNow

Solves a problem with hanging FTP sessions.

Target: 2.8
Require-notes: yes
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6884/
Merge pull request #489 from gbehrmann/fix/2.8/sequentialexecutor

Kill worker when calling SequentialExecutor#shutdownNow