[MLlib] While running regression tests with dense vectors of length greater than 1000, the treeAggregate blows up after several iterations

While running any of the regression algorithms with gradient descent, the treeAggregate blows up after several iterations.

Observed on EC2 cluster with 16 nodes, matrix dimensions of 1,000,000 x 5,000

In order to replicate the problem, use aggregate multiple times, maybe over 50-60 times.

Testing lead to the possible workaround:
setting
`spark.cleaner.referenceTracking false`

seems to help. So the problem is most probably related to the cleanup.