DRILL-5356: Refactor Parquet Record Reader
The Parquet reader is Drill's premier data source and has worked very well
for many years. As with any piece of code, it has grown in complexity over
that time and has become hard to understand and maintain.
In work in another project, we found that Parquet is accidentally creating
"low density" batches: record batches with little actual data compared to
the amount of memory allocated. We'd like to fix that.
However, the current complexity of the reader code creates a barrier to
making improvements: the code is so complex that it is often better to
leave bugs unfixed, or risk spending large amounts of time struggling to
make small changes.
This commit offers to help revitalize the Parquet reader. Functionality is
identical to the code in master; but code has been pulled apart into
various classes each of which focuses on one part of the task: building
up a schema, keeping track of read state, a strategy for reading various
combinations of records, etc. The idea is that it is easier to understand
several small, focused classes than one huge, complex class. Indeed, the
idea of small, focused classes is common in the industry; it is nothing new.
Unit tests pass with the change. Since no logic has chanaged, we only moved
lines of code, that is a good indication that everything still works.
Also includes fixes based on review comments.
closes #789
