LBP: cpp-driver uses first contact point's DC as local DC by default

[muzarski]

Here is a part of the documentation from cpp-driver (cass_cluster_set_load_balance_dc_aware function):

 * Configures the cluster to use DC-aware load balancing.
 * For each query, all live nodes in a primary 'local' DC are tried first,
 * followed by any node from other DCs.
 *
 * <b>Note:</b> This is the default, and does not need to be called unless
 * switching an existing from another policy or changing settings.
 * Without further configuration, a default local_dc is chosen from the
 * first connected contact point, and no remote hosts are considered in
 * query plans. If relying on this mechanism, be sure to use only contact
 * points from the local DC.

We should discuss it here, whether we want to mimic such behaviour in cpp-rust-driver.