MSP: Explicitly store outgoing edges

This lets us avoid the mutex (which was needed because cgraph is not
thread-safe).
MSP: Pass out edges by reference
MSP: A test class wrapper to enable typed tests
MSP: Further preparations for typed tests
MSP: Add Strategy tests
MSP: Rename SequentialMsp -> MspTest
MSP: Merge PriorityLinkedQueue into LinkedQueue

There's no need for this specialization now that the NaiveLinkedQueue is
unused.