ForwardIterator: used newly added VersionStorageInfo::FindFileInRange

[rockeet]

1. FindFileInRange is defined in version_set.cc, ForwardIterator should use it to reduce code explosion
2. The only one FindFileInRange can be optimized some way
   • We have optimized FindFileInRange by devirtualization and prefix caching for known Comparator (now just BytewiseComparator and ReverseBytewiseComparator) with 30x+ speed up, if rocksdb likes this optimization, I'll create a PR.