Add aclose() to BaseCacheBackend and the Redis and Memcached backends

[allen0099]

Only MemoryBackend has aclose(). The lifespan pattern in BACKENDS.md (await backend.aclose()) raises AttributeError on Redis and Memcached, users have to reach into backend.client, and redis-py warns about an unclosed client otherwise.

Proposal

A no-op async def aclose() on BaseCacheBackend; Redis awaits client.aclose(), Memcached runs client.close() in a worker thread. Optionally __aenter__/__aexit__. Document it for every backend.

[allen0099]

Moved to 0.3.9. examples/redis_backend.py (#291) currently closes the client with await backend.client.aclose(). That also needs # type: ignore[attr-defined], because the types-redis stubs predate aclose(). Once AsyncRedisCacheBackend.aclose() exists, switch the example to it and drop the ignore.