OfflineIterator forks ScanDataSource code and doesn't close readers

[DomGarguilo]

From Jira ticket: ACCUMULO-1303

The original ticket states:

createIterator seems to share a bit of code with Tablet, which is just copied instead of reused. This can cause issues down the road if the code in one side changes. If we can try to use the same code base for this logic, it would be better. This may also help with leaving the readers open as well.

It was also pointed out that the code that is shared was eventually moved from Tablet to ScanDataSource.

[DomGarguilo]

Looks like the two sources have diverged pretty far and it may not be possible to cleanly consolidate the two.

I am looking into the second part of the issue now - closing the readers.