Refactor and fix usage of `get_session_index()` and `get_session_info_by_index()` (#4735)

* Rename/refactor around get_session_index

Signed-off-by: Andrei Sandu <[email protected]>

* choose proper head for fetching session

Signed-off-by: Andrei Sandu <[email protected]>

* revert rename

Signed-off-by: Andrei Sandu <[email protected]>

* fix comments

Signed-off-by: Andrei Sandu <[email protected]>

* renaming and more comments

Signed-off-by: Andrei Sandu <[email protected]>

* review feedback

Signed-off-by: Andrei Sandu <[email protected]>

* Run Fetch task in correct session

Signed-off-by: Andrei Sandu <[email protected]>

* Log warning when ancestors unavailable

Signed-off-by: Andrei Sandu <[email protected]>

* Fixes

Signed-off-by: Andrei Sandu <[email protected]>

* fix

Signed-off-by: Andrei Sandu <[email protected]>