use process.emitWarning for user warnings

[trentm]

As a follow-on to both #2001 and the coming elastic/apm-nodejs-http-client#144 let's consider changing the agent to use process.emitWarning when we want to give warnings to the user rather than logger.warn() or logger.error(). That will:

1. allow us to not spam to user if it is a repeated situation (e.g. if we are dropping events because of load on the APM server)
2. make the text more readable for the user (it won't be a JSON log record)
3. allow the user to handle/silence particular warnings if they want

A starter list of specific things we should be warning about:

• demote the log.error about APM server connection issues to a warning (the occasional APM server conn issue is not typically an error the user needs to deal with)
• add a warning if we are dropping a significant number of events because of load to the APM server (this from the maxQueueSize config coming after fix: blocking behaviour under load apm-nodejs-http-client#144

[trentm]

Note that process.emitWarning is now being used in a few places in the agent.