Use lower-entropy random UUIDs

[alecgrieser]

The Java function UUID.randomUUID uses a cryptographically secure random number generator. This can lead to performance bottlenecks for highly concurrent use cases, as there is a lock around the random number generator. However, for effectively all of our random UUIDs (with maybe one exception, in the R-tree), we don't really need secure randomness, and so the lock maintenance is wasted effort.