Added some tests for the Database iterator interface. Updated the post-increment...

Added some tests for the Database iterator interface.  Updated the post-increment operator so that it forks off a copy of the LevelDB or LMDB iterator/cursor when necessary.  Neither of these APIs allow you to directly copy an iterator or cursor, so I create a new iterator and seek to the key that the previous one was currently on.  This means the pre-increment operator can be much cheaper than the post-increment operator.