SPARK-2685. Update ExternalAppendOnlyMap to avoid buffer.remove()

Replaces this with an O(1) operation that does not have to shift over
the whole tail of the array into the gap produced by the element removed.

Author: Matei Zaharia <matei@databricks.com>

Closes #1773 from mateiz/SPARK-2685 and squashes the following commits:

1ea028a [Matei Zaharia] Update comments in StreamBuffer and EAOM, and reuse ArrayBuffers
eb1abfd [Matei Zaharia] Update ExternalAppendOnlyMap to avoid buffer.remove()