Memcache: Honor the expiry parameter in increment() and decrement()

`incrementInternal()` accepts the documented `$expiry` argument but never uses it — `MemcacheIncrementRequest` has no expiration field — so any item created through the `initial_value` path is stored without an expiration and lives until evicted. Callers using `increment()` for rate limiting or short-lived counters accumulate immortal keys, and patterns that rely on expiry to self-heal (e.g. a worker counter that must reset after a crash) never recover.

Start at incrementInternal() and trace how its documented $expiry argument becomes a MemcacheIncrementRequest. Ensure the initial_value path preserves expiration behavior, then verify that increment() and decrement() no longer create immortal keys when an expiry is supplied.

Written by the indexing model from the issue text.