DRILL-5284: Roll-up of final fixes for managed sort
See subtasks for details.
* Provide detailed, accurate estimate of size consumed by a record batch
* Managed external sort spills too often with Parquet data
* Managed External Sort fails with OOM
* External sort refers to the deprecated HDFS fs.default.name param
* Config param drill.exec.sort.external.batch.size is not used
* NPE in managed external sort while spilling to disk
* External Sort BatchGroup leaks memory if an OOM occurs during read
* DRILL-5294: Under certain low-memory conditions, need to force the sort to merge
two batches to make progress, even though this is a bit more than
comfortably fits into memory.
close #761
