HBASE-12550 Check all storefiles are referenced before splitting

Summary: If there are bugs in HDFS move and/or create we should protect against them by making sure that all files referenced end up in split daughters.

Test Plan: Unit tests cover splits pretty well

Subscribers: matteobertozzi

Differential Revision: https://reviews.facebook.net/D29373
HBASE-12550 ADDENDUM Check all storefiles are referenced before splitting