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