Add RNG strengthening (10ms once every minute)
This patch improves the built-in RNG using hash strengthening. At startup, and once every minute, 32 bytes of entropy are produced from the RNG, repeatedly hashed using SHA512 for 10ms, and then fed back into the RNG, together with high-precision timestamps obtained every 1000 iterations.
Backport of core PR15224.
ninja all check-all
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D6139