DRILL-190 (part2)
- MergeJoinBatch handles record batches
- JoinStatus tracks state across input and output batches
- MergeJoinBatchBuilder builds a selection vector of right-side batches which may be rescanned
- implement code stubs for merge join
- add field expression parsing and start of generated merge join code
- code generator support for merge-join's copyLeft(), copyRight(), compare() and compareNextLeftKey()
- add line prefixes to generated code log
- support VectorContainers in declareVectorValueSetupAndMember()
- fix vector allocation in MergeJoinBatch
- fix missing values from left batch when right batch has been exhausted
- fix nullable handling in generated merge join code. make simple merge join test use multiple batches.
- fixes for sv4 batch support, additional multi batch test
