Work around a callback issue and a strange debug-only crash

I'm not sure what's up with the zlib change, but it shouldn't be any less correct.  I thought `close()` was just a deprecated alias for `_close()`, but this nicely avoids the crashes (only when loading symbol maps.)

The callback change allows it to reschedule off the deleted thread for now (while callbacks still do not allow rescheduling off.)

-[Unknown]