error in copy hashtable
Merge branch 'simplify' of https://github.com/krakjoe/apcu
debug-printing instead of warning-printing

When cache->gc_ttl is outdated and reference to item remains,
apc_cache_gc prints warning message.

But this situation should be treated as not warning but debug or notice.
Merge branch 'issues/gc_cache_warning' of https://github.com/cubicdaiya/apcu into simplify