[SPARK-4380] Log more precise number of bytes spilled (1.1)

This is the branch-1.1 version of #3243.

Author: Andrew Or <andrew@databricks.com>

Closes #3355 from andrewor14/spill-log-bytes-1.1 and squashes the following commits:

36ec152 [Andrew Or] Log more precise representation of bytes in spilling code