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.
and... hmm, probably no reason to sync the txn log as well, you can just sync the Presence object alone. Might be cleaner/kinder.