Javadoc
select a split point in the region. The selection of the split point is based on an uniform
distribution assumption for the keys in a region.
Here are some examples:
startKey: aaabcdefg endKey: aaafff split point: aaad
startKey: 111000 endKey: 1125790 split point: 111b
startKey: 1110 endKey: 1120 split point: 111_
startKey: binary key { 13, -19, 126, 127 }, endKey: binary key { 13, -19, 127, 0 },
split point: binary key { 13, -19, 127, -64 }
Set this function as "public static", make it easier for test.