Fix iterator moving backwards and created unit test for the JniDBIterator

[Jaap-Jan]

Iterator follows the following principles

1. Always points to a correct item, unless empty
2. Almost always points to the item to be returned next
3. Except when the last item has been returned. atEnd is set to true at that time and the functions work correctly when the last item has been returned (and the iterator would become invalidated)

[bramklg]

We are having the same issues with the existing iterator. Would be nice if this could be fixed / pulled in the leveldbjni repository.

[davsclaus]

Thanks for this, great work.

[jerrydlamme]

@Jaap-Jan u
Did you run through the DBTest.java ?
Seems a little buggy. Pull request #60 is also complaining about it.

@davsclaus
@chirino
First this commit re-introduce the similar SIGSEGV in issue #40 and test erros happen.
And iterator behavior is re-defined when it moves backward after calling seekToLast().
There are test assertion failures in DBTest.java due to this.
Build fails.

Either we have this commit backed out or fix the issue and update the DBTest.java for new iterator behavior??

[davsclaus]

@jerrydlamme you are welcome to do a PR with a fix.

[jerrydlamme]

@davsclaus
I actually would prefer file a PR to back out this commit first.
Since this commit itself is not a correct fix. The change of seekToLast() is not correct.

leveldb API says:
// Position at the last key in the source. The iterator is
// Valid() after this call iff the source is not empty.
virtual void SeekToLast() = 0;

This commit put the iterator at a fake position after the last key, which is not correct. The moving back
is thus changed, and assertion failure happens.
API behavior cannot be changed this way.
Plus every commit should at least make build clear.

Can you support this revert ?

Thanks

[davsclaus]

yeah sure a PR is welcome