Replace IP allocation logic (#342)

The peer IP allocation logic was allocating sequential peer IP from the 100.64.0.0/10 
address block.
Each account is created with a random subnet from 100.64.0.0/10.
The total amount of potential subnets is 64.
The new logic allocates random peer IP
from the account subnet.
This gives us flexibility to add support for
multi subnet accounts without overlapping IPs.

management/server/testdata/store.json

@@ -21,7 +21,7 @@
                 "Id": "af1c8024-ha40-4ce2-9418-34653101fc3c",
                 "Net": {
                     "IP": "100.64.0.0",
-                    "Mask": "/8AAAA=="
+                    "Mask": "//8AAA=="
                 },
                 "Dns": null
             },