SPARK-785 [CORE] ClosureCleaner not invoked on most PairRDDFunctions

This looked like perhaps a simple and important one. `combineByKey` looks like it should clean its arguments' closures, and that in turn covers apparently all remaining functions in `PairRDDFunctions` which delegate to it.

Author: Sean Owen <sowen@cloudera.com>

Closes #3690 from srowen/SPARK-785 and squashes the following commits:

8df68fe [Sean Owen] Clean context of most remaining functions in PairRDDFunctions, which ultimately call combineByKey
[Minor][Core] fix comments in MapOutputTracker

Using driver and executor in the comments of ```MapOutputTracker``` is more clear.

Author: wangfei <wangfei1@huawei.com>

Closes #3700 from scwf/commentFix and squashes the following commits:

aa68524 [wangfei] master and worker should be driver and executor
SPARK-4814 [CORE] Enable assertions in SBT, Maven tests / AssertionError from Hive's LazyBinaryInteger

This enables assertions for the Maven and SBT build, but overrides the Hive module to not enable assertions.

Author: Sean Owen <sowen@cloudera.com>

Closes #3692 from srowen/SPARK-4814 and squashes the following commits:

caca704 [Sean Owen] Disable assertions just for Hive
f71e783 [Sean Owen] Enable assertions for SBT and Maven build