[SPARK-3885] Provide mechanism to remove accumulators once they are no longer used

Instead of storing a strong reference to accumulators, I've replaced this with a weak reference and updated any code that uses these accumulators to check whether the reference resolves before using the accumulator. A weak reference will be cleared when there is no longer an existing copy of the variable versus using a soft reference in which case accumulators would only be cleared when the GC explicitly ran out of memory.

Author: Ilya Ganelin <ilya.ganelin@capitalone.com>

Closes #4021 from ilganeli/SPARK-3885 and squashes the following commits:

4ba9575 [Ilya Ganelin]  Fixed error in test suite
8510943 [Ilya Ganelin] Extra code
bb76ef0 [Ilya Ganelin] File deleted somehow
283a333 [Ilya Ganelin] Added cleanup method for accumulators to remove stale references within Accumulators.original to accumulators that are now out of scope
345fd4f [Ilya Ganelin] Merge remote-tracking branch 'upstream/master' into SPARK-3885
7485a82 [Ilya Ganelin] Fixed build error
c8e0f2b [Ilya Ganelin] Added working test for accumulator garbage collection
94ce754 [Ilya Ganelin] Still not being properly garbage collected
8722b63 [Ilya Ganelin] Fixing gc test
7414a9c [Ilya Ganelin] Added test for accumulator garbage collection
18d62ec [Ilya Ganelin] Updated to throw Exception when accessing a GCd accumulator
9a81928 [Ilya Ganelin] Reverting permissions changes
28f705c [Ilya Ganelin] Merge remote-tracking branch 'upstream/master' into SPARK-3885
b820ab4b [Ilya Ganelin] reset
d78f4bf [Ilya Ganelin] Removed obsolete comment
0746e61 [Ilya Ganelin] Updated DAGSchedulerSUite to fix bug
3350852 [Ilya Ganelin] Updated DAGScheduler and Suite to correctly use new implementation of WeakRef Accumulator storage
c49066a [Ilya Ganelin] Merge remote-tracking branch 'upstream/master' into SPARK-3885
cbb9023 [Ilya Ganelin] Merge remote-tracking branch 'upstream/master' into SPARK-3885
a77d11b [Ilya Ganelin] Updated Accumulators class to store weak references instead of strong references to allow garbage collection of old accumulators