[SPARK-2554][SQL] CountDistinct partial aggregation and object allocation improvements

Author: Michael Armbrust <michael@databricks.com>
Author: Gregory Owen <greowen@gmail.com>

Closes #1935 from marmbrus/countDistinctPartial and squashes the following commits:

5c7848d [Michael Armbrust] turn off caching in the constructor
8074a80 [Michael Armbrust] fix tests
32d216f [Michael Armbrust] reynolds comments
c122cca [Michael Armbrust] Address comments, add tests
b2e8ef3 [Michael Armbrust] Merge remote-tracking branch 'origin/master' into countDistinctPartial
fae38f4 [Michael Armbrust] Fix style
fdca896 [Michael Armbrust] cleanup
93d0f64 [Michael Armbrust] metastore concurrency fix.
db44a30 [Michael Armbrust] JIT hax.
3868f6c [Michael Armbrust] Merge pull request #9 from GregOwen/countDistinctPartial
c9e67de [Gregory Owen] Made SpecificRow and types serializable by Kryo
2b46c4b [Michael Armbrust] Merge remote-tracking branch 'origin/master' into countDistinctPartial
8ff6402 [Michael Armbrust] Add specific row.
58d15f1 [Michael Armbrust] disable codegen logging
87d101d [Michael Armbrust] Fix isNullAt bug
abee26d [Michael Armbrust] WIP
27984d0 [Michael Armbrust] Merge remote-tracking branch 'origin/master' into countDistinctPartial
57ae3b1 [Michael Armbrust] Fix order dependent test
b3d0f64 [Michael Armbrust] Add golden files.
c1f7114 [Michael Armbrust] Improve tests / fix serialization.
f31b8ad [Michael Armbrust] more fixes
38c7449 [Michael Armbrust] comments and style
9153652 [Michael Armbrust] better toString
d494598 [Michael Armbrust] Fix tests now that the planner is better
41fbd1d [Michael Armbrust] Never try and create an empty hash set.
050bb97 [Michael Armbrust] Skip no-arg constructors for kryo,
bd08239 [Michael Armbrust] WIP
213ada8 [Michael Armbrust] First draft of partially aggregated and code generated count distinct / max

(cherry picked from commit 7e191fe29bb09a8560cd75d453c4f7f662dff406)
Signed-off-by: Michael Armbrust <michael@databricks.com>