Native UUID and JSON serialization

• UUIDs are now generated as UUIDv4. When available, the native randomUUID() from the Crypto API is used; otherwise the library falls back to a simple custom UUIDv4 implementation.
• When available, the library now consistently uses native JSON.stringify() for serializing request data, only falling back to the older custom JSON encoding implementation if the environment doesn't have native JSON APIs.
• Fixes a Session Recording race condition where sometimes the idle timeout is reset past when it should have fired due to a backgrounded tab.