stablerange: move a utility function around It make more sense to have this small function earlier in the series

stablerange: remove the now unused individual range class That class is now longer necessary, we dropped its usage for performance reason.

stablerange: directly use tuple to refer to a stable range Now that all advance logic lives in the unified class we no longer needs the individual class. Creating and operating on this cache introduce a significant overhead that we can not stop having. From now on, a range a is pair tuple '(headrev, index)'. Long live the tuple.

obshash: properly cache obshash value The code was buggy and only cached non-nullid/non-inherited value. This was previous hidden because we were using a 'propertycache' to cache the value on the range object. This fixes it and restore a lot of performance.