v2 of dual maxres engine

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>
mss and mss-mus-maxres

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>
fix bug in blocked clause elimination that was enabled for external variables, fix other bugs in maxres

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>