Use fastrand instead of prands

[neilalexander]

In order to avoid top-level lock contention in the math/rand package, we were previously initialising our own prand sources in various structs. However these still needed to be protected by locks and they also just weren't that fast.

This PR removes those prands and replaces those calls with calls to the Go runtime's fastrand, which is a couple hundred times faster and is, more importantly, lock-free:

goos: darwin
goarch: arm64
pkg: github.com/nats-io/nats-server/v2/server/fastrand
BenchmarkFastRand32
BenchmarkFastRand32-10          1000000000               0.2564 ns/op
BenchmarkFastRand64
BenchmarkFastRand64-10          1000000000               0.5019 ns/op
BenchmarkDefaultRand32
BenchmarkDefaultRand32-10       11997840               104.4 ns/op
BenchmarkDefaultRand64
BenchmarkDefaultRand64-10       11745747               105.8 ns/op
PASS
ok      github.com/nats-io/nats-server/v2/server/fastrand       3.768s

Signed-off-by: Neil Twigg neil@nats.io

[derekcollison]

LGTM

[wallyqs]

LGTM