Some american fuzzy lop tests managed to replace resources, however the
old values got leaked (I am not sure if this should be allowed)

In all cases, introduce a destroy callback to the hashtable, used when a
value is removed, replaced or the table is cleared. This simplifies a
bit resource management and help avoid potential errors by using simply
refcounts.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>