actors: Rename *clean-up* back to *cleanup.

By popular intarwebs opinion.

* 8sync/actors.scm (<actor>, <hive>, hive-handle-cleanup-all)
(self-destruct, run-hive): Rename all references of clean-up to cleanup.
(hive-handle-cleanup-all, run-hive-cleanup): Rename name from clean-up
version.
* tests/test-actors.scm: Likewise.