[SPARK-3085] [SQL] Use compact data structures in SQL joins

This reuses the CompactBuffer from Spark Core to save memory and pointer
dereferences. I also tried AppendOnlyMap instead of java.util.HashMap
but unfortunately that slows things down because it seems to do more
equals() calls and the equals on GenericRow, and especially JoinedRow,
is pretty expensive.

Author: Matei Zaharia <matei@databricks.com>

Closes #1993 from mateiz/spark-3085 and squashes the following commits:

188221e [Matei Zaharia] Remove unneeded import
5f903ee [Matei Zaharia] [SPARK-3085] [SQL] Use compact data structures in SQL joins