Fix #772: Speed up `DateTime.to_clock_time` (#782)
Fix #772: Speed up `DateTime.to_clock_time` (#782)

Speedup is around x100 to x200. Since this function is used when serializing
DateTime objects to Bolt, this also affects the driver's speed when handling
certain DateTime objects.
neo4j/neo4j-python-driver,neo4j/neo4j-python-driver