Backport RelaxedAtomic<> to 3.0.x

The RelaxedAtomic<> class was added in the watson branch to add an atomic type with relaxed memory consistency, suitable for things like statistical counters.

As part of fixing a number of data races on ep-engine/3.0.x detected by ThreadSanitizer we want to backport a number of patches from master & watson to 3.0.x to fix the issues. However some of those patches depend on RelaxedAtomic, therefore we need to backport that class to 3.0.x