Use a local FastRandomContext in a few more places in net

Summary: This is a backport of Core PR14624 : https://github.com/bitcoin/bitcoin/pull/14624/commits/8098379be5465f598220e1d6174fc57c56f9da42

Test Plan:

make check

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3447