Fix #2279: Use concurrent data structures in SummaryReport.

[nicolasstucki]

No description provided.

[felixmulder]

Are you sure this fixes the issue? I'd like to have seen a thread dump with the original issue.

[nicolasstucki]

I haven't been able to reproduce and the dump at the moment where a concurrent update breaks the coherency on the ArrayBuffer. But I got the following array after adding cleanups with append.

Where the implementation of append is:

def +=(elem: A): this.type = {
  ensureSize(size0 + 1)
  array(size0) = elem.asInstanceOf[AnyRef]
  size0 += 1
  this
}