actors: Update self-destruct to run *clean-up* message handler.

* 8sync/actors.scm (self-destruct): Wait on *clean-up* action before
self-destructing by default.
(run-hive): Add parameter to specify whether or not to clean-up after
the hive finishes its tasks.
* tests/test-actors.scm: Add tests for disabling #:clean-up on run-tasks
and ensuring that *clean-up* runs on self-destruct.