Add xchacha20 stream cipher.
Allow PSK reusing.

- Now KDF uses xchacha20 for PRP, that allows 24 bytes nonce
- Nonce is now inserted to the hpenc header
- For all PSK initial nonce is generated randomly, allowing 2^24 bytes
  of entropy to be used, making thus nonce repeating probability as
  negligible
Update block per key to 4096 from 1024.