Reconsider Parquet read buffer size

[jlowe]

ParquetPartitionReaderBase uses the parquet.read.allocation.size config to control the copy buffer used for reading from the Parquet input stream. The default is 8MB which is pretty big, especially when there are many reader threads via the multithreaded reader. With a large number of reader threads all performing Parquet reads, these per-thread buffers can add up to a nontrivial amount of heap memory which can trigger OOMs. We should do some empirical measurements in various environments to see if we can significantly lower the default for this.

[sameerz]

Duplicate of issue #9269