Update test protos for PingStream method

[emcfarlane]

Test protos were using CumSum as a Ping method that returned twice and all other methods were unused. Regenerate the PingService with a PingStream methods that returns a full duplex method of Ping. Size is removed from the constant 2 to be calculated with proto.Size() (response sizes are the same as request, so response isn't calculated).

Testing changes:

• CumSum == PingStream but always returns a response for a request, not two responses for each request
• failPing returns an error, not a message than an error.

No behaviour should be changed.

[akshayjshah]

Left some questions and suggestions, but they're optional. You're welcome to address the comments by pushing code here and then merging it (no more review needed), in follow-up PRs, or by sending my thoughts directly to /dev/null :)