Instead of casting lease ID to uint32, fix the TTL() function
to use etcd time-to-live API for determining TTL. Add configurable
min-lease-ttl and max-lease-ttl options to prevent extreme TTL
values. By default, lease records now go through bounds checking
with 30s to 1d as the min/max.
Added unit tests for validation and docs.
Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>
