This patch change groupByKey() to use external sort based approach, so it can support single huge key.

For example, it can group by a dataset including one hot key with 40 millions values (strings), using 500M memory for Python worker, finished in about 2 minutes. (it will need 6G memory in hash based approach).

During groupByKey(), it will do in-memory groupBy first. If the dataset can not fit in memory, then data will be partitioned by hash. If one partition still can not fit in memory, it will switch to sort based groupBy().

Author: Davies Liu <davies.liu@gmail.com>
Author: Davies Liu <davies@databricks.com>

Closes #1977 from davies/groupby and squashes the following commits:

af3713a [Davies Liu] make sure it's iterator
67772dd [Davies Liu] fix tests
e78c15c [Davies Liu] address comments
0b0fde8 [Davies Liu] address comments
0dcf320 [Davies Liu] address comments, rollback changes in ResultIterable
e3b8eab [Davies Liu] fix narrow dependency
2a1857a [Davies Liu] typo
d2f053b [Davies Liu] add repr for FlattedValuesSerializer
c6a2f8d [Davies Liu] address comments
9e2df24 [Davies Liu] Merge branch 'master' of github.com:apache/spark into groupby
2b9c261 [Davies Liu] fix typo in comments
70aadcd [Davies Liu] Merge branch 'master' of github.com:apache/spark into groupby
a14b4bd [Davies Liu] Merge branch 'master' of github.com:apache/spark into groupby
ab5515b [Davies Liu] Merge branch 'master' into groupby
651f891 [Davies Liu] simplify GroupByKey
1578f2e [Davies Liu] Merge branch 'master' of github.com:apache/spark into groupby
1f69f93 [Davies Liu] fix tests
0d3395f [Davies Liu] Merge branch 'master' of github.com:apache/spark into groupby
341f1e0 [Davies Liu] add comments, refactor
47918b8 [Davies Liu] remove unused code
6540948 [Davies Liu] address comments:
17f4ec6 [Davies Liu] Merge branch 'master' of github.com:apache/spark into groupby
4d4bc86 [Davies Liu] bugfix
8ef965e [Davies Liu] Merge branch 'master' into groupby
fbc504a [Davies Liu] Merge branch 'master' into groupby
779ed03 [Davies Liu] fix merge conflict
2c1d05b [Davies Liu] refactor, minor turning
b48cda5 [Davies Liu] Merge branch 'master' into groupby
85138e6 [Davies Liu] Merge branch 'master' into groupby
acd8e1b [Davies Liu] fix memory when groupByKey().count()
905b233 [Davies Liu] Merge branch 'sort' into groupby
1f075ed [Davies Liu] Merge branch 'master' into sort
4b07d39 [Davies Liu] compress the data while spilling
0a081c6 [Davies Liu] Merge branch 'master' into groupby
f157fe7 [Davies Liu] Merge branch 'sort' into groupby
eb53ca6 [Davies Liu] Merge branch 'master' into sort
b2dc3bf [Davies Liu] Merge branch 'sort' into groupby
644abaf [Davies Liu] add license in LICENSE
19f7873 [Davies Liu] improve tests
11ba318 [Davies Liu] typo
085aef8 [Davies Liu] Merge branch 'master' into groupby
3ee58e5 [Davies Liu] switch to sort based groupBy, based on size of data
1ea0669 [Davies Liu] choose sort based groupByKey() automatically
b40bae7 [Davies Liu] bugfix
efa23df [Davies Liu] refactor, add spark.shuffle.sort=False
250be4e [Davies Liu] flatten the combined values when dumping into disks
d05060d [Davies Liu] group the same key before shuffle, reduce the comparison during sorting
083d842 [Davies Liu] sorted based groupByKey()
55602ee [Davies Liu] use external sort in sortBy() and sortByKey()