support load balanced CNAME records

[allenxwang]

For example, we would like to read AWS route53 records with multiple CNAMEs.

multi-value (load-balanced) ResourceRecordSets:

Tasks:

• [x] implement base tests for round-robin cnames in mock
• [ ] implement round-robin cnames in ultradns
• [ ] implement round-robin cnames in route53
• [ ] implement round-robin cnames in dynect

[codefromthecrypt]

in UltraDNS, the following commands are needed and imply a premium account: addTCLBPool, addPoolRecord, updatePoolRecord, getPoolRecords, getLoadBalancingPoolsByZone

[codefromthecrypt]

I've a proposal to expose the following to help folks understand the valid values they can place for weight.

List<Integer> WeightedRecordSetApi.supportedWeights();

This is a List as opposed to a DiscreteDomain or Range, as in the case of UltraDNS the values are not contiguous. RangeSet could be used, but that's awkward, especially if exporting as a ReST call. The downside of List is that we need to include every possible number. Since the upper bound is 256 elements (route53), it seems pragmatic to use a list to describe all valid weights.

Here are the constraints that make the case.

• in all providers, weight = 0 means always serve.
• in DynECT, valid values are 0-15
• in UltraDNS, valid values are 0-100 (even numbers)
• in Route53, valid values are 0-255

cc @allenxwang @colmmacc @jonbodner @jdamick