[SPARK-8579] [SQL] support arbitrary object in UnsafeRow

This PR brings arbitrary object support in UnsafeRow (both in grouping key and aggregation buffer).

Two object pools will be created to hold those non-primitive objects, and put the index of them into UnsafeRow. In order to compare the grouping key as bytes, the objects in key will be stored in a unique object pool, to make sure same objects will have same index (used as hashCode).

For StringType and BinaryType, we still put them as var-length in UnsafeRow when initializing for better performance. But for update, they will be an object inside object pools (there will be some garbages left in the buffer).

BTW: Will create a JIRA once issue.apache.org is available.

cc JoshRosen rxin

Author: Davies Liu <davies@databricks.com>

Closes #6959 from davies/unsafe_obj and squashes the following commits:

5ce39da [Davies Liu] fix comment
5e797bf [Davies Liu] Merge branch 'master' of github.com:apache/spark into unsafe_obj
5803d64 [Davies Liu] fix conflict
461d304 [Davies Liu] Merge branch 'master' of github.com:apache/spark into unsafe_obj
2f41c90 [Davies Liu] Merge branch 'master' of github.com:apache/spark into unsafe_obj
b04d69c [Davies Liu] address comments
4859b80 [Davies Liu] fix comments
f38011c [Davies Liu] add a test for grouping by decimal
d2cf7ab [Davies Liu] add more tests for null checking
71983c5 [Davies Liu] add test for timestamp
e8a1649 [Davies Liu] reuse buffer for string
39f09ca [Davies Liu] Merge branch 'master' of github.com:apache/spark into unsafe_obj
035501e [Davies Liu] fix style
236d6de [Davies Liu] support arbitrary object in UnsafeRow

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