ds: guard against stack-not-refcounted quirk of Perl 5

The Perl 5 stack is weakly-referenced for performance reasons.
This means it's possible for items in the stack to be freed
while executing further down the stack.

In lei (and perhaps public-facing read-only daemons in the
future), we'll fork and call PublicInbox::DS->Reset in the child
process.  This causes %DescriptorMap to be clobbered, allowing
the $DescriptorMap{$fd} arg to be freed inside the child
process.

When Carp::confess or Carp::longmess is called to generate a
backtrace, it may access the @DB::args array.  This array access
is not protected by reference counting and is known to cause
segfaults and other weird errors.

While the caller of an unnecessary Carp::confess may be
eliminated in a future commit, we can't guarantee our
dependencies will be free of @DB::args access attempts
in the future.

So guard against this Perl 5 quirmk by defensively bumping the
refcount of any object we call ->event_step on.

cf. https://rt.perl.org/Public/Bug/Display.html?id=131046
    https://github.com/Perl/perl5/issues/15928

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