Implement an Expiring Record Store

Implement a level-based in-memory record store. The store contains records identified by a string `key`. Each record contains fields, where each `field` is a string and each `value` is a string. ### Level 1: Basic mutations Implement: - `set(key, field, value)`: Create the record if it does not exist, then set `field` to `value`. - `get(key, field)`: Return the current value for `field`, or `null` if the record or field does not exist. - `delete(key, field)`: Remove `field` from the record. Return `true` if a field was removed, otherwise return `false`. If a record becomes empty, it may be removed from the store. ### Level 2: Scanning Implement: - `scan(key)`: Return all fields for `key`, sorted lexicographically by field name. Format each item as `field(value)`. Return an empty list if the record does not exist. - `scan_by_prefix(key, prefix)`: Return only fields whose names start with `prefix`, sorted lexicographically by field name and formatted as `field(value)`. ### Level 3: Timestamps and TTL Extend the store so fields can be written with timestamps and optional time-to-live values. Implement: - `set_at(key, field, value, timestamp)`: Set the field at `timestamp` with no expiration. - `set_at_with_ttl(key, field, value, timestamp, ttl)`: Set the field at `timestamp`; it is visible for query times `t` such that `timestamp <= t < timestamp + ttl`. - `delete_at(key, field, timestamp)`: Delete the field at `timestamp` if it exists and is not expired at that time. Return `true` if a field was deleted, otherwise return `false`. - `get_at(key, field, timestamp)`: Return the value visible at `timestamp`, or `null` if none exists. - `scan_at(key, timestamp)`: Return all fields visible at `timestamp`, sorted and formatted as in `scan`. - `scan_by_prefix_at(key, prefix, timestamp)`: Return all visible fields at `timestamp` whose names start with `prefix`, sorted and formatted as in `scan_by_prefix`. Assume timestamps passed to timestamped operations are non-decreasing. A later write to the same `(key, field)` replaces the previous value and expiration policy. Expired or deleted fields must not appear in reads or scans.