Distributed Tracing by Default: Interpret Collector Response

[RichVanderwal]

The collector is sending a new limit per the spec to allow the span reservoir size to change.

Here are the changes to the spec since we started putting DT by Default into it: https://source.datanerd.us/agents/agent-specs/compare/08a02381f577a37beb40137ed19155a15344503c...master