Use std::atomic in boost
Use std::atomic in boost

Summary:
By default, boost its own atomic, but this is unecessary becasue we are using C++11 .

Backport of core's PR10239

Test Plan:
  ./autogen.sh
  ./configure
  make check

Reviewers: schancel, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D868

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