Average stats should be calculated in Nanoseconds but exposed in Milliseconds

ISPN-7768 changed the precision of the exposed average-<read|write>time stats from Milliseconds to Nanoseconds. This causes backwards compatibility issues with previous versions of Infinispan and JDG. Therefore we need to ensure that these stats are exposed to the user in Milliseconds, however they should still be calculated in Nanoseconds internally to improve accuracy and avoid rounding errors.