Fix invalid use a memory order relaxed

Summary: It is self evident that these operation need to have a global order, or two thread may end up allocating the same id. Thankfully, the x86 memory model ensures that it won't happen, but I wouldn't bet as much on various flavour of ARM or other RISC CPUs.

Test Plan: Read carefully, I guess...

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5662