[SPARK-4772] Clear local copies of accumulators as soon as we're done with them

I suspect this PR is not really quite ready for merging, but this seemed the best way of getting a few comments on it.

A little explanation:
I'm working with some rather large accumulators.  I've been asked why not just use a reducer, and the answer is, that when this works, it works about 10x as fast.

And so far, when it doesn't work, it seems improvable.  This is my first attempt at such improvement.

As far as I can tell, on each worker, when each time a task is run, the following happens:
1. The global accumulator object is cleared on that thread
2. The current individual accumulators are registered on that thread (as copies)
3. When the task completes, the accumulator values are collected and returned.

Note that when this is done, nothing is cleared.  This means, if one is using large accumulators, those accumulators just use up memory uselessly until the next task is run on that thread.

And if a thread somehow dies, that memory is used up and can't be retrieved.

And, as far as I can tell, that local thread value will never be used again.

All I've done so far is to clear local values for that thread as it returns them.

I would like to do 2 more things:
1. Reduce this to a two-step process: register, and return, both clearing values
2. Put in something to make sure values are cleared if a task dies (or a thread too, if that is possible)
I can do those on this PR, or if someone wants to just incorporate this PR now, I can make a new one for them - as far as I can tell, this one-line change does no harm.

This seems so simple so far, though, I was hoping that I could get some confirmation that I understood things correctly before going on.

I don't have a PR or JIRA issue on this so far - the issues I see are a bit hard to pin down.  When I run my application without this change, it works, but is very fragile - some tasks take 6 seconds, some 4 minutes.  When I implement this, everything seems to run smoothly, and I stop getting random, large task times.

This is somewhat related to JIRA issue SPARK-664 in that they both deal with accumulator performance, but it doesn't really address that directly.
Not related to your changes and I don't expect you to fix it, but this could be an `AtomicInteger` instead.