Fixes #8538 - ISC dhcp plugin now respets binding

This is wrong, you should do `subnet.leases.find`. If you have a lease file with both a reservation and an expired lease for the same IP, then this will delete both. An example lease file would be:

```
host quux.elysium.emeraldreverie.org {
  dynamic;
  hardware ethernet 52:54:00:31:a3:97;
  fixed-address 172.20.10.53;
        supersede server.filename = "pxelinux.0";
        supersede server.next-server = ac:14:0a:01;
        supersede host-name = "quux.elysium.emeraldreverie.org";
}
lease 172.20.10.53 {
  starts 0 2014/04/06 15:01:31;
  ends 0 2014/04/06 16:00:09;
  tstp 0 2014/04/06 16:00:09;
  cltt 0 2014/04/06 15:01:31;
  binding state free;
  hardware ethernet 00:1d:60:a5:b1:2a;
}
```

Using `subnet.records`, the reservation will be parsed, added to `subnet` and then the lease parsing will hit line 107 (because state is 'free') and come here via line 108. This method will return the reservation, and the it will be deleted, even though it's a valid reservation.

Could you fix and add a test?