Cache Stats improvement

[Matthias Herzog]

optionally collect cache stats.

const cache = Velo.builder().stats().build();
const stats = cache.stats();
cache.stats() returns a CacheStatistics object
export interface CacheStatistics {
  hits: number;
  misses: number;
  hitRatio: number;
}

if not enabled the stats should be zero or throw an error at cache.stats()