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.

changelog: drop the side_write argument to revlog splitting The only user is now gone.

changelog: stop useless enforcing split at the end of transaction Changelogs are no longer created inline, and existing changelogs are automatically split. Since we now enforce splitting at the start of any write, we don't need to enforce splitting at the end of the transaction. This has the nice side effect of killing the only user of "side_write".

tests: don't use "status" operand of dd in test-censor.t (issue6858) Some implementations don't have this operand, let's just direct stderr into /dev/null, that's pretty cross-platform. Also specify bs=512 (the default for me), because the default might be different on different systems. Other uses of dd in the tests do specify it, so this is more consistent.