feat(schedule): two-phase commit job scheduling

After an Engine schedules a Job, the target Agent responds
by clearing the TTL from the /.../target key in etcd. The
TTL set by the Engine happens to be 0 for now, but it will
be nonzero at a future time.
fix(schedule): Use nonzero in two-phase commit

Actually use a nonzero value when an Engine persists
a scheduling decision to etcd.

Fix #500.