Node.js agent v6.4.1

Improvements

Removed calls to OutgoingMessage.prototype._headers in favor of using public getHeaders API (thanks to @adityasabnis for bringing this to our attention).
Removed engine upper-bound to enable easier experimentation of newer Node versions with the agent for customers.
Please see https://docs.newrelic.com/docs/agents/nodejs-agent/getting-started/compa... for officially supported versions. Incompatibilities are expected for odd-numbered releases, which are not supported, and even-numbered releases before "official" support has been released.
Reduced "... Aggregator data send." log messages to debug level to reduce noise of default logs.
Prevented passing CI with .only() in mocha tests.
Removed CI restriction for Node 12.15. Node shipped a fix for the 12.16 breakage in 12.16.1.
Reduced logging level of raw x-queue-start or x-request-start header values to avoid logging very large values at default logging levels.

Bumped @newrelic/aws-sdk version to v1.1.2 from v1.1.1. https://github.com/newrelic/node-newrelic-aws-sdk/blob/master/CHANGELOG.md
Notable improvements include:
- Fixed issue where instrumentation would crash pulling host and port values when AmazonDaxClient was used as the service for DocumentClient.
Fixed issue where disabled agent would return an empty string instead of an empty object from API getLinkingMetadata().
This issue would cause the @newrelic/winston-enricher module to crash when attempting to inject log metatdata.

If a .NET agent is initiating distributed traces as the root service, you must update that .NET agent to version 8.24 or later before upgrading your downstream New Relic Node agents to this agent release.