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 <andrei-mihail@parity.io>

* choose proper head for fetching session

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* revert rename

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* fix comments

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* renaming and more comments

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* review feedback

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* Run Fetch task in correct session

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* Log warning when ancestors unavailable

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* Fixes

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>

* fix

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>