core (grpc) use retry strategy

[stephenplusplus]

Fixes #1265

To Dos

• Unit tests

// @callmehiphop

[stephenplusplus]

@callmehiphop this is just another option, didn't mean for it to auto-invalidate your work. If you don't think this approach is the right way to go, just let me know! I'll wait before I get the tests ready.

[callmehiphop]

It's cool, this was the first response I'd seen from you in regards to this issue in several days, so I figured you weren't a fan of what I was propsing. I originally did start going down this route, but I felt like the nesting/wrapping increased the cyclomatic complexity, which is why I decided to abandon it. If you think this is the best way to go, that's fine with me.

[stephenplusplus]

It actually wasn't that I wasn't a fan, it was just hard to review from the 🚗 😢 👶 situation. Like you, I'm okay with all, but not a super-fan of any. I like this overall because it didn't force changes to retry-request (which I thought would be good originally, but in practice, turns out awkward [as you predicted]). I also like this because it doesn't duplicate any retry logic which already exists in retry-request. I'll get going on some tests.

[coveralls]

Coverage remained the same at 100.0% when pulling 51da084 on stephenplusplus:spp--1265 into e9e4170 on GoogleCloudPlatform:master.