This PR converts the hard-deletion interval to a timeout:
- to prevent the interval from not being restored when hard deletion
throws, and
- to prevent a long-running hard deletion from leading to duplicate
deletions.
This fixes a bug in the pruning (soft-delete). The pruning was a bit too
aggressive, as it also pruned executions that weren't in an end state
yet. This only becomes an issue if there are long-running executions
(e.g. workflow with Wait node) or the prune parameters are set to keep
only a tiny number of executions.
