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()
SPARK-2711. Create a ShuffleMemoryManager to track memory for all spilling collections

This tracks memory properly if there are multiple spilling collections in the same task (which was a problem before), and also implements an algorithm that lets each thread grow up to 1 / 2N of the memory pool (where N is the number of threads) before spilling, which avoids an inefficiency with small spills we had before (some threads would spill many times at 0-1 MB because the pool was allocated elsewhere).

Author: Matei Zaharia <matei@databricks.com>

Closes #1707 from mateiz/spark-2711 and squashes the following commits:

debf75b [Matei Zaharia] Review comments
24f28f3 [Matei Zaharia] Small rename
c8f3a8b [Matei Zaharia] Update ShuffleMemoryManager to be able to partially grant requests
315e3a5 [Matei Zaharia] Some review comments
b810120 [Matei Zaharia] Create central manager to track memory for all spilling collections