Use ACPu storage for monotonic counters

[lroman242]

This Pull Request addresses the issue of monotonic counters in multi-process environments (such as PHP-FPM):

• Moved timestamp state management from class static properties to APCu shared memory.
• Updated getUnixTimeSubsec() and getUnixTimeMs() to use apcu_fetch and apcu_store.
• This ensures that UUID versions 6, 7, and 8 maintain strict monotonicity across different PHP-FPM worker processes, preventing potential collisions or out-of-order sequences during high concurrency.
• Removed obsolete static properties: $unixts, $subsec, and $unixts_ms.