chg: nil: Add optional debugging output for fetch context reference counting

As the fetch context reference counting was converted to userspace RCU
reference counting, the ability to debug the reference counting was
lost.  Restore the debugging by adding the optional compile-time enabled
debugging output again.

Merge branch 'ondrej/add-tracing-to-fctx-reference-counting' into 'main'

See merge request isc-projects/bind9!11230