IPv6 support

[fracappa]

Currently, the implemented load balancer logic supports only IPv4 traffic.

Support for IPv6 should be implemented as well.

Currently, the session key is only designed for IPv4 traffic:

struct flow_key {
    __be32 src_ip;
    __be32 dst_ip;
    __be16 src_port;
    __be16 dst_port;
    __u8   protocol;
} __attribute__((packed));

It needs to be changed to something like the following to support also IPv6:

struct flow_key {
    uint8_t ip_version;  
    uint8_t protocol;
    union {
             __be32  src_ipv4;
             __be32 src_ipv6 [4];
     } src_addr;
    union {
           __be32  dst_ipv4;
           __be32 dst_ipv6[4];
   } dst_addr;
    __be16 src_port;   
    __be16 dst_port;    
} __attribute__((packed)); 

Also the remaining logic should be adjusted accordingly as IPv6 packets have different headers than IPv4 ones.