gRPC follow ups

[benthecarman]

• Use gRPC status code consts for client: Replace REST and RabbitMQ with gRPC #176 (comment)
• Bias select toward shutdown in event stream: Replace REST and RabbitMQ with gRPC #176 (comment)

[ldk-reviews-bot]

👋 Thanks for assigning @tnull as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[tnull]

Thanks!