state/machine: use WaitAgentPresence, rename AgentAlive

Using WaitAgentPresence after SetAgentPresence gives time for the first write of the pinger data to mongo to be persisted. This fixes issues with juju status and ensure-availability being run in quick succession with juju bootstrap.
add a TODO to point out that this doesn't help with multiple state servers, and that we need to trigger a sync on those as well, but it's coming later