Extract Stripe superclass from StripeSM

[JosiahWI]

The StripeSM class retains all the evacuation data and the mutex.

[cmcfarlen]

What is the incentive to do this? Are there plans to use the split out Stripe class on its own in some way? It seems like without the lock from Continuation, this can't be used on its own.

Maybe consider making Stripe or some superclass of Stripe an interface to clean that up and have StripeSM implement the aspects that integrate with Continuations and locking.

[JosiahWI]

This is an code organization change. The long term incentive is separation of concerns, and the short term goal is to split up massive source files into smaller, separate source files. In this case, we're separating the data structure from the event handling. The separation is still subpar because the event handling methods touch the data structure too much, but it's a start. This will be followed by a PR to move all the StripeSM event handling into a new StripeSM.cc source file.