revlog: move the splitting-inline-revlog logic inside the inner object This is another large IO block that we need to move within the inner object if we want's it to be self sufficient.

revlog: synchronise the various attribute holding the index filename The segmentfile and the attribute need to be synchronized, let's enforce that.

revlog: drop reference to docket in the inline-splitting code revlog with a docket do not use inline revlog and do not need to split them. So we can remove some code handling docket there.

revlog: move _getsegmentforrevs on the internal object See inline documentation for details.

revlog: create a iteration of a _InnerRevlog object within the revlog The goal of this object is to isolate a sub-API that can be implemented by a compiled object (e.g. Rust). So the boundary of this object will be arbitrary depending of what can we easily implemented in the Compiled code. For now, we start simple, and move the code that manage the IO objects in the inner object. More will come in the coming changesets. Note: the object definition could live in the different module to thin the `revlog.py` file, however there are other better candidate for extraction first and I have enought patch stacked on top of the this one for the split in this patch not to be worth it. So I leave this to future me.

revlog: drop the unused `_chunkcache` attribute Apparently, some time ago, the chunk cache moved in the randomaccessfile object.

revlog: drop the unused `_sidedatareadfp` method It has no caller anywhere and is probably the remains of some older code.

revlog: also migrates `revlog.upperboundcomp` to ConfigClass This was planned but overlooked when doing the rest of the migration.

revlog: small doc to the `files` method

revlog: remove the `_indexfp` method The method is only used two time and we want to move that code to a lower level object. So we simply inline the method to simplify further refactoring.