sd-dhcp-server: clear out expired leases when processing requests

sd-dhcp-server: clear out expired leases when processing requests

The DHCP server configuration supports setting a maximum lease time, but old
leases are never actually cleared out if the client doesn't send a RELEASE.
This causes the pool to run out of addresses on networks where clients just
disappear, which is a fairly common occurrence on wireless networks.

Fix this by cleaning up expired leases before processing client requests, so
addresses can be reused for new clients.