Made SimpleMemoryCheck thread-safe

[Dr15Jones]

Switched SimpleMemoryCheck to use the new Service APIs. In addition, switched to using std::atomics for thread safety. The module was changes so that if a callback is made while another callback is updating the memory measurement, the second callback just returns immediately.

[cmsbuild]

A new Pull Request was created by @Dr15Jones (Chris Jones) for CMSSW_7_0_X.

Made SimpleMemoryCheck thread-safe

It involves the following packages:

FWCore/Services

@smuzaffar, @Dr15Jones, @ktf, @nclopezo can you please review it and eventually sign? Thanks.
@wmtan this is something you requested to watch as well.
You can sign-off by replying to this message having '+1' in the first line of your reply.
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@ktf you are the release manager for this.

[Dr15Jones]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next IBs unless changes or unless it breaks tests. @ktf can you please take care of it?

[nclopezo]

+1
the errors in the unit tests are the ones that were already in the IB:
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/1056/summary.html

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next IBs unless changes or unless it breaks tests. @ktf can you please take care of it?