Refactor btree iterator last index

[cheme]

After working on #124.
I did realize many variants of LastIndex were useless.
This PR changes:
LastIndex::Start -> LastIndex::Before(0)
LastIndex::End -> LastIndex::Before(last separator + 1) . (so can be equal to ORDER).
LastIndex::After(i) -> LastIndex::Before(i + 1)

Also a seek operation do not need a direction.
And seek_to_last is now using a variant of SeekTo enum.

Close #124

[Tpt]

Thank you!

[Tpt]

Sadly, I believe I there is still an issue. If we have a value >= ORDER in the LastIndex::At value, the following line will fail because there are only ORDER separators:

parity-db/src/btree/iter.rs

Line 430 in 3e988a7

if let Some(address) = state.1.separator_address(at) {

(I discovered this problem while fuzzing this change)

[cheme]

Thank you, I also see other issue with seek, I will fix this.

[cheme]

I ended up refactoring (see updated description) a bit but should be a good thing.

[Tpt]

Thank you for the update!

I have fuzzed it during 1/2h and it seems to work properly now.