Boost Bind Message Silencing, master branch (2021.01.25.)

I've seen the following kinds of messages in our builds since forever:

In file included from /build/atnight/localbuilds/nightlies/Athena/master/athena/Control/AthenaKernel/src/AlgorithmTimer.cxx:34:
/cvmfs/sft.cern.ch/lcg/releases/Boost/1.73.0-3c166/x86_64-centos7-gcc8-opt/include/boost/bind.hpp:41:1: note: #pragma message: The practice of declaring the Bind placeholders (_1, _2, ...) in the global namespace is deprecated. Please use <boost/bind/bind.hpp> + using namespace boost::placeholders, or define BOOST_BIND_GLOBAL_PLACEHOLDERS to retain the current behavior.

These are not recognised by the nightly system as warnings (since they aren't really), but they sort of are. See for instance:

http://atlas-computing.web.cern.ch/atlas-computing/links/distDirectory/gitwww/MASTERWebArea/ardoc_web_areamaster64BC7G8AthenaOpt/ARDOC_Log_2021-01-24T2101/Control_AthenaKernel.html

I tried to do the minimal amount of changes necessary for silencing these messages. Though it would be worth thinking about possibly harmonising how we use JSON in our C++ code. Since in a number of places we use nlohmann/json by now, should that be treated as the default way of using JSON files? 🤔