TTL cache

Implement a TTL cache. Process operations of three shapes: ["set", now, key, value, ttl] stores key=value at time now with a time-to-live of ttl (the entry expires at now+ttl, i.e. it is alive for times strictly less than now+ttl), ["get", now, key] returns the value if the key exists and now < its expiry, else -1, and ["count", now] returns how many keys are currently alive at time now. Setting an existing key overwrites its value and resets its expiry. Timestamps are non-decreasing non-negative integers. Return the list of results from every get and count operation, in order.