oxd and kong clustering

[yuriyz]

This requires additional development. oxd creates so called "site
files" which is actually session. If kong node1 communicates with oxd1
and kong node2 with oxd2 then we have problem if request served by
node1 will go to node2.

For this case I think we need to push oxd "site json" to cassandra in
the same way as kong does it.

See this https://getkong.org/docs/0.6.x/clustering/

In case we keep oxd data in cassandra we are on safe side then:

1. each kong node can have own oxd or can even share oxd between nodes
2. all data are replicated among nodes by cassandra.

[yuriyz]

scheduled for 3.2 future version since it's not urgen cc: @nynymike

[yuriyz]

oxd supports redis. Also we can add other persistence if needed. Then we should open ticket which should say what exact persistence we wish to support.