revlog: make `reading` not crash on empty repository If the revlog is empty, the file might not exist and the open will fails. This is not great, but that details or this is now contained in the revlog itself.

changelog: make the `_delayopener` a real proxy We use the same approach as the `_divertopener`. This make it possible to use other methods of the vfs and not just the `__call__` API.

perf: ensure all readlog's reading is done within a `reading` context We are about to enforce this at the revlog level, so we update the perf code in advance.

perf: change the way we approach revlog reading If the `reading` context manager is available, we should use it over explicit file handle management. This will help us to make file handle management a matter more internal to the revlog.

revlog: document the `rev` method A small docstring does not hurt.

revlog: document the `clearcaches` method A small docstring does not hurt.

revlog: document the `update_caches` method A small docstring does not hurt.

revlog: make the `candelta` method private It is only used internally, so lets make it clear.

revlog: document the `get_streams` method A small docstring does not hurt.

revlog: document the `is_inline_index` method A small docstring does not hurt.