Add ServiceStartExecutable, ServiceStopExecutable, ServiceStartRevertible, and ServiceStopRevertible. Delete Service interface.
Add tests for enabling/disabling registries/services in separate transactions (as these new tests will reproduce different scenarios, internally, than the corresponding pre-existing tests).
Test tidy up: remove print statement from RollbackServiceStateTestCase, and properly ident statement at AbstractTransactionTest
WIP making UndemandDependencies revertible (working on making this revertion work properly)