This patch does following:
1. Planner changes to create ChunkDispatch node when MERGE command
has INSERT action.
2. Changes to map partition attributes from a tuple returned from
child node of ChunkDispatch against physical targetlist, so that
ChunkDispatch node can read the correct value from partition column.
3. Fixed issues with MERGE on compressed hypertable.
4. Added more testcases.
5. MERGE in distributed hypertables is not supported.
6. Since there is no Custom Scan (HypertableModify) node for MERGE
with UPDATE/DELETE on compressed hypertables, we don't support this.
Fixes#5139