start simplifying index code and make exclusive work

[phritz]

separate logic commits:

• make exclusive work
• remove scan key index (offset)
• encode index keys ourselves and simplify IndexKey

TODO:

• eliminate ScanOptionsInternal and friends
• flatten ScanOptions for convenience

This feels a lot better to me than what we had before. @arv this changes the interface but i would wait for me to land next set of changes before doing the work to switch over. I'm going to simplify ScanOptions a bit.

Progress towards #250