A custom RegionSplitPolicy implementing a SplitPolicy that groups
rows by a prefix of the row-key with a delimiter. Only the first delimiter
for the row key will define the prefix of the row key that is used for grouping.
This ensures that a region is not split "inside" a prefix of a row key.
I.e. rows can be co-located in a region by their prefix.
As an example, if you have row keys delimited with _, like
userid_eventtype_eventid, and use prefix delimiter _, this split policy
ensures that all rows starting with the same userid, belongs to the same region.