[SPARK-7190] [SPARK-8804] [SPARK-7815] [SQL] unsafe UTF8String

Let UTF8String work with binary buffer. Before we have better idea on manage the lifecycle of UTF8String in Row, we still do the copy when calling `UnsafeRow.get()` for StringType.

cc rxin JoshRosen

Author: Davies Liu <davies@databricks.com>

Closes #7197 from davies/unsafe_string and squashes the following commits:

51b0ea0 [Davies Liu] fix test
50c1ebf [Davies Liu] remove optimization for upper/lower case
315d491 [Davies Liu] Merge branch 'master' of github.com:apache/spark into unsafe_string
93fce17 [Davies Liu] address comment
e9ff7ba [Davies Liu] clean up
67ec266 [Davies Liu] fix bug
7b74b1f [Davies Liu] fallback to String if local dependent
ab7857c [Davies Liu] address comments
7da92f5 [Davies Liu] handle local in toUpperCase/toLowerCase
59dbb23 [Davies Liu] revert python change
d1e0716 [Davies Liu] Merge branch 'master' of github.com:apache/spark into unsafe_string
002e35f [Davies Liu] rollback hashCode change
a87b7a8 [Davies Liu] improve toLowerCase and toUpperCase
76e794a [Davies Liu] fix test
8b2d5ce [Davies Liu] fix tests
fd3f0a6 [Davies Liu] bug fix
c4e9c88 [Davies Liu] Merge branch 'master' of github.com:apache/spark into unsafe_string
c45d921 [Davies Liu] address comments
175405f [Davies Liu] unsafe UTF8String