[PATCH 01 of 14] cache: introduce an abstract class for cache we can upgrade incrementally

[Jun Wu]

Excerpts from Boris Feld's message of 2017-07-10 15:22:19 +0200:
> Jun, could you take a look at the 7 first changesets and give us your
> feedback about the obsstoresourcebase class, is it helpful for your
> work on radixlink?

I think it's over complicated for my usecase. With the obsstore format
change, it could be 2 lines to detect strip, and another a few lines to use
length to incrementally build the index.

If both index and obsstore are native code for performance, then Python code
cannot be reused.