sd-dhcp-client: make sure timers fire immediately

The default slack caused there to be a delay before timers fired. Solve it
by setting timers that should trigger immediately to trigger far in the past.

This brings down the ideal-case dhcp lease acquisition time from about 500ms to
about 50ms (over a veth pair, so no network latency involved).

All the rest of the time (except for ~0.5ms) is spent in the bind() call in,
dhcp_network_bind_raw_socket(). I don't know if there is anything to be done
about that though...