manager: add new intermediate state for stop->start transitions

manager: add new intermediate state for stop->start transitions

We always check for re-starting a service if its in the started
state, but for those that go from a (request_)stop to the stopped
state it can be useful to explicitly have a separate transition.

The newly introduced `request_start` state can also be used for CRS
to opt-into starting a service up on a load-wise better suited node
in the future.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>