revlog: move the `_chunk` method on the inner object This is a necessary step before being able to move more logic around restoring a revision content there. For now, we do a simple patch for the perf extension logic, when the implementation of the inner object changes, we will likely need some evolution of the API. However this is true of many things in the perf extension. So we will see this later.

revlog: move the compression/decompression logic on the inner object This is a necessary step before being able to move more logic around restoring a revision content there. For now, we do a simple patch for the perf extension logic, when the implementation of the inner object changes, we will likely need some evolution of the API. However this is true of many things in the perf extension. So we will see this later.

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.