Various improvements to state transitions.

Various improvements to state transitions.

Introduce a new variable to track if a service is waiting for its
dependencies (starting) or dependents (stopping). In these cases
it is possible to transition directly from STARTING to STOPPED or
from STOPPING to STARTED. This also removes the need for special
handling of "internal" services (which will never transition from
STARTING to STARTED or STOPPING to STOPPED excet via transitions
of their dependencies/dependents).