Previously, the ehcache statistics were computed on the activity of the
last minute which gives a "live" overview. All others cache managers,
including JCache, provides a "cumulative" metrics (i.e. the hit/miss
ratio since the creation of the cache or the last time it got cleared).

Ths commit aligns the ehcache statistics to provide a similar semantics
as the other cache managers. The side effect is that the metrics are now
available, even if there is no cache activity at all at the moment.

Closes gh-4891