delta-find: move target_rev in the _DeltaSearch.__init__ Now that we have an object we can initialize that attribute at initialization time.

delta-find: move snapshot_cache in the _DeltaSearch.__init__ Now that we have an object we can initialize that attribute at initialization time.

delta-find: move `_rawgroups` on the `_DeltaSearch` object Moving more code before doing more logic changes.

delta-find: move `_refinedgroups` on the `_DeltaSearch` object Moving more code before doing more logic changes.

delta-find: introduce a _DeltaSearch object That object represent the search of a good delta for one revision. It will replace the interleaved generator currently in use. It will make the logic more explicit and easier to split into different subclass for the algorithm variant. We will move content gradually before doing deeper rework. For now, we only move the `_candidategroups` function here. More will follow in the same series.

delta-find: add a small docstring to deltacomputer As we are about to introduce another object related to finding delta. So lets have a minimal docstring to the existing one.

revlog: stop using `atomictmp` for the split revlog Since we already manually deal with writing on the side and delaying visibily, we no longer need this.