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.

shelve: drop some weird manually stripping before transaction abort I cannot find a case where this is actually useful, so lets triggering strip in the middle of uncommitted transaction.