Add time based UUIDs.

Incorporate some of the changes from @kimchy and @s1monw.
Move the UUID generators into their own classes and provide a common
interface as a first step to moving them under a singleton. Use a better
method of getting the mac address and fall back to a secure random address
if it fails. Add tests to test conccurency and shared prefix integrity of
UUIDs. Use PaddedAtomicLongs to hold the sequence number and lasttime.
Check to see if a time slip has occured as described by @s1monw in a CAS loop.
Next step is to move the impls under a singleton.

See #5941