Changelog entry for Trace Context.
### New Features
++ Distributed Tracing now supports W3C Trace Context headers. When distributed
+ tracing is enabled with `Config.DistributedTracer.Enabled = true`, the agent
+ will now accept W3C's `traceparent` and `tracestate` headers when calling
+ [`Transaction.AcceptDistributedTraceHeaders`](https://godoc.org/github.com/newrelic/go-agent/v3/newrelic#Transaction.AcceptDistributedTraceHeaders).
+ When calling
+ [`Transaction.InsertDistributedTraceHeaders`](https://godoc.org/github.com/newrelic/go-agent/v3/newrelic#Transaction.InsertDistributedTraceHeaders)
+ the agent will include the W3C headers along with the New Relic distributed
+ tracing header. To disable insertion of the New Relic distributed trace
+ header (while keeping the W3C headers) use the
+ `Config.DistributedTracer.InsertNewRelicHeader` configuration field.
+
* Event data is now sent to New Relic every five seconds, instead of every
minute. As a result, transaction, custom, and error events will appear in
near-realtime within APM and Insights.
