Design a key-value store that supports versioning through timestamps. The problem is divided into three parts with increasing complexity.

Part 1: Basic Key-Value Store

Implement a basic key-value store with set and get operations.

Example:

KV kv = new KV();
kv.set("foo", "bar");
kv.get("foo");  // Returns "bar"

Part 2: Versioned Key-Value Store

Extend the basic store to support versioning. The set operation returns a timestamp, and get can optionally take a timestamp to retrieve historical values.

Example:

KV kv = new KV();
long timestamp = kv.set("foo", "bar");
Thread.sleep(1000);  // Wait 1 second
kv.set("foo", "bar2");

// Get value at specific timestamp
kv.get("foo", timestamp);  // Returns "bar"

// Get latest value
kv.get("foo");  // Returns "bar2"

Part 3: Time-Based Version Retrieval

Extend Part 2 to support getting values at any point in time, not just exact timestamps. When given a timestamp, return the most recent value that was set before that timestamp.

Example:

KV kv = new KV();
long timestamp = kv.set("foo", "bar");
Thread.sleep(1000);
kv.set("foo", "bar2");

// Get latest value
kv.get("foo");  // Returns "bar2"

// Get value at timestamp + 750ms
kv.get("foo", timestamp + 750);  // Returns "bar"