[SPARK-6075] Fix bug in that caused lost accumulator updates: do not store...

[SPARK-6075] Fix bug in that caused lost accumulator updates: do not store WeakReferences in localAccums map

This fixes a non-deterministic bug introduced in #4021 that could cause tasks' accumulator updates to be lost.  The problem is that `localAccums` should not hold weak references: after the task finishes running there won't be any strong references to these local accumulators, so they can get garbage-collected before the executor reads the `localAccums` map.  We don't need weak references here anyways, since this map is cleared at the end of each task.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #4835 from JoshRosen/SPARK-6075 and squashes the following commits:

4f4b5b2 [Josh Rosen] Remove defensive assertions that caused test failures in code unrelated to this change
120c7b0 [Josh Rosen] [SPARK-6075] Do not store WeakReferences in localAccums map