LoadIncrementalHFiles should be able to handle CFs with blooms

When LoadIncrementalHFiles loads a store file that crosses region boundaries, it will split the file at the boundary to create two store files. If the store file is for a column family that has a bloom filter, then a "java.lang.ArithmeticException: / by zero" will be raised because ByteBloomFilter() is called with maxKeys of 0.

The included patch assumes that the number of keys in each split child will be equal to the number of keys in the parent's bloom filter (instead of 0). This is an overestimate, but it's safe and easy.