Updates Protobufs message and conversion for IPSubnet

The patch also enhances IPv6 support for IPAddress by:
- including a common IPVersion enum in the Protobufs message
definition
- adding support for the IPv6Addr class

Change-Id: I5b5be8642d3598a5f46cdb42738e61b3a56fd4ac
Signed-off-by: Alex Bikfalvi <alex.bikfalvi@midokura.com>