plugins/ipam: round robin ip allocation for host-local ipam

This changes the ip allocation logic to round robin. Before this, host-local IPAM searched for available IPs from start of subnet. Hence it tends to allocate IPs that had been used recently. This is not ideal since it may cause collisions.

plugins/ipam/host-local/backend/store.go

@@ -21,6 +21,7 @@ type Store interface {
 	Unlock() error
 	Close() error
 	Reserve(id string, ip net.IP) (bool, error)
+	LastReservedIP() (net.IP, error)
 	Release(ip net.IP) error
 	ReleaseByID(id string) error
 }