Legacy comparator in Hfile trailer will fail to load

HBASE-24968 moved MetaCellComparator out from CellComparatorImpl to avoid the deadlock issue. But this introduced compatibility issue, old hfile with comparator class as "org.apache.hadoop.hbase.CellComparator$MetaCellComparator" will fail to open due to ClassNotFoundException.

Also we should also handle the case when comparator class is "org.apache.hadoop.hbase.CellComparatorImpl$MetaCellComparator", which was case before HBASE-24968.