Support BinaryType in UnsafeRow, just like StringType.

Also change the layout of StringType and BinaryType in UnsafeRow, by combining offset and size together as Long, which will limit the size of Row to under 2G (given that fact that any single buffer can not be bigger than 2G in JVM).

Author: Davies Liu <davies@databricks.com>

Closes #6911 from davies/unsafe_bin and squashes the following commits:

d68706f [Davies Liu] update comment
519f698 [Davies Liu] address comment
98a964b [Davies Liu] Merge branch 'master' of github.com:apache/spark into unsafe_bin
180b49d [Davies Liu] fix zero-out
22e4c0a [Davies Liu] zero-out padding bytes
6abfe93 [Davies Liu] fix style
447dea0 [Davies Liu] support binaryType in UnsafeRow