DRILL-6791: Scan projection framework
The "schema projection" mechanism:
* Handles none (SELECT COUNT\(*)), some (SELECT a, b, x) and all (SELECT *) projection.
* Handles null columns (for projection a column "x" that does not exist in the base table.)
* Handles constant columns as used for file metadata (AKA "implicit" columns).
* Handle schema persistence: the need to reuse the same vectors across different scanners
* Provides a framework for consuming externally-supplied metadata
* Since we don't yet have a way to provide "real" metadata, obtains metadata hints from
previous batches and from the projection list (a.b implies that "a" is a map, c[0]
implies that "c" is an array, etc.)
* Handles merging the set of data source columns and null columns to create the final output batch.
* Running tests found a failure due to an uninialized "bits" vector. Added code to explicitly fill
the bits vectors with zeros in the "result set loader."
