Add randomness pool mode for V4 UUID and NullUUID for SQL Scanners.

From Andrey Pechkurov:

Adds an optional randomness pool mode for Random (Version 4) UUID generation. The pool contains random bytes read from the random number generator on demand in batches. Enabling the pool may improve the UUID generation throughput significantly.

Since the pool is stored on the Go heap, this feature may be a bad fit for security sensitive applications. That's why it's implemented as an opt-in feature.

From Samuel Roth:

Added support for NullUUID

A NullUUID can be marked not valid (i.e., null) for use with JSON and the database/sql/driver.Scanner interfaces.