Mirage_crypto_rng_lwtSourceRNG seeding on Lwt.
This module initializes a Fortuna RNG with getrandom(), and CPU RNG.
initialize ~sleep () will bring the RNG into a working state. The argument sleep is measured in ns (default 1s), and used to sleep between collection of entropy from the CPU RNG, every 10 * sleep getrandom is used to collect entropy.