Currently GrandpaBlockImport only supports importing one pending authority set change, it will error when importing a second block that signals an authority set change if any previous one has not been finalized yet. This can be problematic if finality is temporarily stalled, as authorities will keep producing blocks but will stall after a second authority change is signaled (e.g. at the end of the session).

GrandpaBlockImport should allow multiple non-overlapping changes on the same branch. In order to do it safely all change blocks must be justified and the justifications must be imported in-order (so that authority set changes are enacted accordingly).