Reimplement persistence logic using precise allocation rather than memory pools. This reduces
memory usage of cache entries, especially for small values, and improves performance of persisting
and unpersisting values.
Fixed GH #335: Stampede protection is broken.
Fixed GH #328: Segfault in apcu_key_info() if APCu is disabled.
Generally make the behavior of functions if APCu is disabled more consistent.
Fixed PHP bug #72980: Empty strings are now consistently allowed as cache keys.
Optimized apcu_key_info() and apcu_cache_info() by using interned strings.