Very rough draft.  Barely works.

By casting to SafeLinkIntrusiveListHook if appropriate, this fixes a
bug in the erase() method, where erase() calls
IntrusiveListHook::unlink() instead of
SafeLinkIntrusiveListHook::unlink().

This reduces delays of zero-duration timers from up to 1 second to
zero.  libavahi-client schedules zero-duration timers often.

The "std::nested_exception" catch block already covers this perfectly.

Similar to boost::intrusive::safe_link.

This will allow using Zeroconf in output plugins (preparing for
Snapcast with Zeroconf).