Use FRC::randbytes instead of reading >32 bytes from RNG

Summary:
There was only one place in the codebase where we're directly reading >32 bytes from
the RNG. One possibility would be to make the built-in RNG support large reads, but
using FastRandomContext lets us reuse code better.

There is no change in behavior here, because the FastRandomContext constructor
uses GetRandBytes internally.

Partial backport of Core PR14955
https://github.com/bitcoin/bitcoin/pull/14955/commits/6a57ca91da23c6a5d91399ffc7fc09a99b6d4c76

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4387