Add time based UUIDs

See #5941.


Using SecureRandom as a UUID generator is slow and doesn't allow us
to take adavantage of some lucene optimizations around ids with common
prefixes. This commit will allow us to use a
timestamp64bit-macAddr-counter UUID. Since the macAddr may be shared
among several nodes running on the same hardware we use an xor of the
macaddr with a SecureRandom number generated on startup.
Also, I wonder if we should randomize the sequenceID on init, to try to thwart the "machine/JVM goes down, clock goes backwards, JVM restarted" case ...