DC-aware discovery and load balancing

[hubertpompecki]

Work in progress. Addresses https://github.com/lexhide/xandra/issues/199.

Introduces :autodiscover_other_datacenters option

When false, Xandra will ignore topology change events describing new nodes in datacenters it never opened control connections to. When true, Xandra will open connections to all new nodes regardless of which datacenter they are started in. The default is false.

Introduces :dc_aware load balancing strategy

The strategy can only be used when autodiscovery option is set to true. With this strategy, Xandra will prefer sending requests to nodes located in datacenters it opened control connections to. Datacenters are tried in the order control nodes are listed in :nodes. Nodes are chosen in a random fashion within a given datacenter. If there aren't any connections with a datacentre matching :nodes anymore Xandra will choose a connection randomly.

To do:

• [ ] Tests
• [ ] Moduledocs