Add comprehensive cache statistics

The `CacheStats` class provides detailed metrics like hit/miss counts,
load success/failure counts, total load time, and eviction counts.
It also offers derived metrics such as hit/miss rates, load failure rate,
and average load penalty. The design is inspired by Caffeine.

`BasicClientSideCache` now uses a `StatsCounter` to accumulate these
statistics, exposed via a new `stats()` method. The previous
`cacheHits()` and `cacheMisses()` methods have been removed.

A `recordStats` option (default: true) in `ClientSideCacheConfig`
allows disabling statistics collection.

Author: bobymicroby