Add sinter to cache, plus dead_letter and get_queue_depth to messaging backends

[tanmay-s-lyzr]

Summary
Three capability gaps in the abstraction, across cache and messaging:

1. Cache set intersection (sinter) is missing. The other set ops (sadd/srem/smembers/scard/sismember) were added on feat/redis_set_functions, but sinter was never included — and it has never existed in any prior commit/release (git log --all -S sinter returns nothing; there is no v0.2.2). This corrects the earlier assumption that set ops were "dropped in 0.2.2" — they were net-new, not a regression.
2. MessagingBackend.dead_letter(receipt_handle, reason) — no way to route a poison message to a dead-letter queue. Azure has a native per-message API; SQS does not and must emulate it.
3. MessagingBackend.get_queue_depth() — no way to read the approximate
   number of waiting messages (needed for autoscaling / backlog monitoring /
   alerting).