Error level for the case of what cache is outdated and its reference remains

When ``cache->gc_ttl`` is outdated and reference to cache item remains,

**apc_cache_gc** prints warning message. ("GC cache entry '%s' was on gc-list for %d seconds")

But this situation should be treated as not warning but debug or notice.

Bacause this situation often occurs for real and PHP & APCu still can continue without problem.