Allow superseding the lease renewal and rebind times.

Also make sure that the renewal is never more than 1/2 * expiry and
rebind never more than 7/4 * renewal (the default values in the spec).
This should allow adjusting high values from the server as well as
making sure the values from the server make sense.

Renewal and rebind times will be adjusted down if the expiry time is set
very high in a server, not the other way around. This change just makes
sure the values keep making sense.