Create a TTL (Time-To-Live) cache. Entries expire after ttlMs milliseconds. Accepts an optional clock function for testing. Give maxEntries a value to bound the cache: storing a new key at the cap drops the oldest-stored entry first. Bound any cache whose keys can be chosen from outside (e.g. unknown session cookies), so a flood of unique keys cannot grow it without limit. Leave it unset for caches whose keyspace our own data already bounds — the keyed entity caches rely on holding every row of their table.