Fix the exporter-ocagent

[justindsmith]

The exporter-ocagent had two critical issues which left the exporter unusable:

1. The required grpc proto files were not being referenced correctly (Node package folder location assumptions are broken #174)
2. The ConfigStream behavior (where the ocagent can send configuration changes down to the client) was not connecting / receiving correctly from the ocagent, causing errors and spamming logs

The following changes have been applied to fix the above issues:

1. We now package the proto files for both OpenCensus and the google common proto set into the final @opencensus/exporter-ocagent package. The changes mostly reside in the package.json configuration.
2. We have removed all references to the ConfigStream behavior. This is a breaking change, but the ocagent exporter was not in a usable form previously, so I don't expect there to be any users directly impacted.

A few other changes have been included as well:

• The default ocagent port has been changed to 55678 to match the default OC Agent/Collector port.
• A slight reconnect delay has been added to avoid rapid reconnect attempts.
• Updated grpc dependency version to 1.18.0.
• Readme / Docs update
• Changelog update

These changes have all been tested locally against a real local ocagent instance and been confirmed to be working.

[draffensperger]

Should there be an issue to track adding the ConfigStream behavior back?

[mayurkale22]

LGTM. If possible, please include exporter-ocagent example in examples folder?

[justindsmith]

LGTM. If possible, please include exporter-ocagent example in examples folder?

I took a quick look and didn't see any tracing specific examples. I don't think having an exporter specific example is useful without the larger tracing context, so I'm going to hold off on adding an example, but I'll add a tracking issue to add a tracing-specific example, as I think that would be useful.