Thoughts on replacing inheritance of TrSessionImpl with composition/delegation

[Jack Singleton]

I've done a spike for TopologyMaintenanceSessionImpl, switching from
inheritance to composition.

This simplified some of the unit test code, because we can mock methods on
TrSession instead of each of the methods that TrSession calls.

I think this will mean we can start changing how
TopologyMaintenanceSessionImpl manages dependencies without changing how
all session implementations work.

Thoughts?

https://github.com/sanity/tahrir/compare/topology-maintenance-session-refactor

Jack