Reactive eviction strategy

[jonastheis]

In #293 eviction is not yet fully handled. We need to ensure that the protocol, commitments and chains are properly evicted.

It also has an influence on how we subscribe to reactive properties as these can become nil in the tear down process, which necessitates a nil check in many handlers. See here for reference.

Some more pointers:

• how to load commitments: Feat: Reactive protocol rework #293 (comment)
• initEviction in Protocol: Feat: Reactive protocol rework #293 (comment)
• children referencing parents and cleaning themselves up: Feat: Reactive protocol rework #293 (comment)