DRILL-1887: Add code-gen for explicitly comparing null values for hash table keys and make corresponding change for hash aggregation and hash join.

Add test cases for joins and aggregations on nullable columns.
DRILL-1824: Fix equality between maps and lists for test framework.

Fix failing test due to not creating hadoop's Text object, as well as lack of equality between these Text objects and a string containing the same text.
DRILL-1900: Fix numeric overflow problem in hbase stat calculation.