DRILL-7601: Shift column conversion to reader from scan framework
Allows the column writers to be generic, moves scan-specific
conversions into each reader where needed, implemented in
a reader-specific way.
Adds a revised way of handling projections in the result set
loader that is not coupled with conversion, as the prior
design was.
Updates the CSV, Avro, Log and HDF5 readers.
closes #1993
