dispute-coordinator: fix earliest session checks for pruning and import (#6358)

* RollingSession: add fn contains

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

* handle_import_statements fix ancient dispute check

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

* work with earliest session instead of latest

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

* update comment

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

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