Make session ID's Base64 with leading timestamp

Session ID's gain a leading timestamp to ensure they
are generally monotonic, then they are URL-safe, Base64
encoded with the SHA1 digest.
Add optional support for crypto-strength session ID's

If Math::Random::ISAAC::XS and Crypt::URandom are installed, the
seeds for the session ID's will be cryptographically strong.
Restore speed advantage of crypto session tokens

All the Moo indirection was too costly.  This more direct
closure-based approach restores the crypto speed advantage, while
still letting the ISAAC algorithm be seeded late after pre-forking.

With crypto enabled, I benchmarked ID generation more than twice
as fast as non-crypto ID generation (284350/s vs 98641/s)
Merge pull request #164 from dagolden/topic/improve-session-token

Improve session tokens