fix #6388: silently discard tasks when shutting down executor
fix #6388: silently discard tasks when shutting down executor

This will work only for executors that we do control explicitly,
such as the network and the refresh executors.

matej116/cgeo,pstorch/cgeo,S-Bartfast/cgeo,auricgoldfinger/cgeo,Bananeweizen/cgeo,superspindel/cgeo,Bananeweizen/cgeo,superspindel/cgeo,kumy/cgeo,tobiasge/cgeo,cgeo/cgeo,auricgoldfinger/cgeo,matej116/cgeo,tobiasge/cgeo,tobiasge/cgeo,cgeo/cgeo,rsudev/c-geo-opensource,kumy/cgeo,matej116/cgeo,pstorch/cgeo,rsudev/c-geo-opensource,rsudev/c-geo-opensource,kumy/cgeo,superspindel/cgeo,S-Bartfast/cgeo,Bananeweizen/cgeo,pstorch/cgeo,cgeo/cgeo,S-Bartfast/cgeo,auricgoldfinger/cgeo,cgeo/cgeo