Expression::evaluate() is called from many sites that have not yet been wired to a stage-level tracker. By adding a default fallback tracker we ensure that every evaluation is bounded irrespective of the path that triggered it. A new knob will be added `internalQueryMaxSingleExpressionMemoryUsageBytes` that will bound the memory usage when the fallback tracker is used. This knob will have no effect when a tracker has been passed by the caller.