Change Writer/Readers factory to create CellComparators rather than KVComparator

In HBASE-10800 we would try to create CellComparators so that later when we deal with cells we could add the comparison logic into the comparators of those cells. This JIRA is aimed at changing the writers and readers factory methods where we always accept a KVComparator. It would be better if we could create a CellComparator.