Add an Option<>al InvoiceFeatures object for the payee in get_route

Add an Option<>al InvoiceFeatures object for the payee in get_route

We currently only use it to override the graph-specific features
returned in the route, though we should also use it to enable or
disable MPP.

Note that tests which relied on MPP behavior have had all of their
get_route calls upgraded to provide the MPP flag.