KeyPrefixRegionSplitPolicy and DelimitedKeyPrefixRegionSplitPolicy splits are not always optimal

With KeyPrefixRegionSplitPolicy (and now DelimitedKeyPrefixRegionSplitPolicy), if a split would break a group of keys it is modified to become the first key in the group's range. This is not always optimal. If the distribution of keys are such that the group containing the split has half the keys in the region, then no split will occur.

The best solution would be to compute both the current key group's first key (what the current implementation does) as well as the next key group's first key and then choosing which of the two is closest to the original split point.