1) When DateTimeFormatter is reused we don't need to copy availableZoneIds and allocate nonRegionIds as PrefixTree can be taken from cache. In the related benchmark allocation of HashSet takes ~93% of all time, so avoiding it should bring some improvement for cases when we reuse DateTimeFormatter to parse a string into ZoneDateTime.

2) DateTimeFormatter is mostly used with one locale, so cachedTree and cachedTreeCI can have predefined size.