title: MapDB collection size issue
body: <p>I'm using MapDB 0.9-snapshot. I have some collection containing several millions of records String->String.
  My problem is that the method size() on such ConcurrentMap(s) takes a lot of time and a lot of memory (I have even an OutOfMemory error).</p>
  
  <p>Any other experienced such a problem?
  Thanks
  Bye</p>