Add randomness pool mode for V4 UUID

[puzpuzpuz]

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.

Please treat this PR as a performance experiment.

Benchmarks

Environment: Ubuntu 20.04, go version go1.16.2 linux/amd64, i5-8300h

master branch:

BenchmarkUUID_New-8   	          597717	        1797 ns/op	      16 B/op	       1 allocs/op

This branch:

BenchmarkUUID_New-8   	          632412	        1809 ns/op	      16 B/op	       1 allocs/op
BenchmarkUUID_NewPooled-8   	 2058459	       579.7 ns/op	       0 B/op	       0 allocs/op

The BenchmarkUUID_NewPooled benchmark here stands for the pool mode. The pool size could be increased from current 256 bytes to gain a slightly better throughput, but that would come at the cost of larger memory footprint.

[pborman]

Over all this is fine, just need to add the note about non-threadsafe calls.

[puzpuzpuz]

@pborman thanks for the review!

[pierrre]

Hi !
Just a quick question:
why not use bufio.Reader ?
Is it to heavy/complex ?

[puzpuzpuz]

why not use bufio.Reader ?
Is it to heavy/complex ?

@pierrre
TBH I didn't consider bufio.Reader when writing the code. The logic seems to be pretty straight-forward to me, so I kept the code simple and used a plain array + a position variable. But I guess, bufio.Reader would also work just fine.