Use signals to call Importer and Distributor cancel methods.
Added a few more unit tests for our new signal handlers.
Rename graceful_cancel to register_sigterm_handler.
Remove an unused import.
Merge branch 'distributed-tasks' into rbarlow-cancel_syncs

Conflicts:
	server/test/unit/server/async/test_tasks.py
	server/test/unit/server/managers/repo/test_sync.py