WIP refactoring var length column readers
WIP moving stuff around to make the code easier to read, all tests are passing except the decimal reader, going to explore the commit history to see if I can find the change that broke it.

WIP - doesn't appear that decimal test was actually broken, just failing for the same reason as before with an issue of not clearing the Ve value vector buffer, but this has a patch outstanding for it.
WIP - moving the metadata conversion and column reader creation out of the ParquetRecordReader into their own classes