Add LoggedAtomic<> debug class

Add LoggedAtomic<> debug class

A Debugging wrapper around std::atomic which print all accesses to the
atomic value to stderr.

Drop-in compatible with AtomicValue for simple use-cases - currently
only implements load() / store().

Change-Id: I78cec4d8bad55588900573f201d81a38f16f97ee
Reviewed-on: http://review.couchbase.org/67944
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Daniel Owen <owend@couchbase.com>