See WT-13672 for additional context

Currently cache_state_gen_max_gap will only be reported when WiredTiger is configured with `statistics=cache_walk`, but this configuration is CPU intensive so we rarely enabled it.

We should also report this stat as part of the normal "fast" statistics. We should maintain the same computation of (evict->evict_pass_gen - page->evict_pass_gen), but fetch this value when the eviction server find the page and adds it the eviction queue. The stat should store the maximum value seen but occasionally rest the value to zero. See "maximum page size seen at eviction" for an existing example.

I'll leave it to the developer to determine if this should be a new stat, or if it replaces the existing cache_state_gen_max_gap stat which is gated behind cache_walk