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.
I know this was pre-existing, but it's (1) a little bit nasty to read and (2) ignores errors.

    pinger1 := s.setAgentAlive(c, "1")
    defer assertKill(c, pinger1)

?