Make lease expiry times truly unique.

Turns out arc4random_uniform(1000000) isn't enough to guarantee no two
leases expire on precisely the same time.  Pointed out by perfdhcp from
ISC Kea which is too slow to stress this server as it consumes 100% CPU
on my laptop after about 7000 requests/second.  Peaks went over 20k rps
and dhcpd was well below 50% CPU on AMD FX-8320.  Yes, that is with all
the logging we do (syslogd went to about 20%, but on a different core).

Signed-off-by: Martin Pelikan <pelikan@storkhole.cz>