Bulk Operation

IMPALA-4835 Consider reserving max 2 I/O buffers per Parquet column when there are multiple columns

IMPALA-4835 Don't claim ideal reservation in scanner until actually processing scan ranges

IMPALA-4835 Better estimate of per-column compressed data size for low-NDV columns.

IMPALA-4835 Investigate memory requirement regression from scanner buffer pool change

IMPALA-4835 Various tests failing (some flaky) with memory limit exceeded

IMPALA-4835 Fix test gap - no test of handling for invalid/unsupported Parquet codec

IMPALA-4835 Crash in DiskMgr::AllocateBuffersForRange

IMPALA-4835 Simplify ScannerContext buffer management to only use one I/O buffer at a time.

IMPALA-4835 Consider limiting I/O buffer queue size to 2 buffers

IMPALA-4835 Consider always copying-out Disk I/O buffers instead of attaching to RowBatches