revlog: minor refactor in the chunk gather process We will introduce some caching in this method in the next changeset, we make some of the most "disruptive" change first as touching this could break (and maybe did during the development process).

changelog-delay: move the delay/divert logic inside the (inner) revlog Instead of hacking throught the vfs/opener, we implement the delay/divert logic inside the `_InnerRevlog` and `randomaccessfile` object. This will allow to an alternative implementation of the `_InnerRevlog` that does not need to use Python details. As a result, the new implementation can use the transaction less agressively and avoid some extra output since no data had been written yet. That seems like a good side effect.

revlog: add a `canonical_index_file` attribute on inner revlog This is currently the same and the index_file but it will become more complex when we start doing delayed write.

changelog-delay: move "delayed" check to a more official API To avoid reaching inside the inner object in the future, we needs some official API. We put one of such API early to reduce the size of the final diff.

changelog-delay: move the appender class next to randomaccessfile We want to move the delay/divert logic at the revlog level (to have all IO related logic in the _InnerRevlog) we start with small piece that are easy to move on their own.

changelog-delay: adds some check around delaying and diverting write Theses assert shows we never call delay or divert if the revlog hold file handle on the revlog's file.

revlog: consolidate cache invalidation within the inner objet The invalidation of data that belong to the inner object can and should be delegated to the inner object.