core: implement retry strategy for grpc requests

[callmehiphop]

Closes #1265

This adds a retry with backoff strategy for gRPC requests. Currently it will only attempt to retry for gRPC error code 14 (service unavailable).

@heidmotron @leonardpunt @jmdobry if any of you would like to test if this resolves your issues, that would be awesome :)

[stephenplusplus]

Is it possible to use retry request directly? It accepts a custom request function. If it's close, but kind of a hack, maybe it just needs a pr to generalize it.

[callmehiphop]

I initially did go down this route but it started to get pretty hacky, so I gave up on it. However if you think a PR there is the way to go, I'd be more than happy to try and tackle that.

[stephenplusplus]

If it's possible without getting into hack territory, I think that would be awesome.

[coveralls]

Coverage remained the same at 100.0% when pulling 072602b on callmehiphop:grpc-retry-requests into e9e4170 on GoogleCloudPlatform:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 072602b on callmehiphop:grpc-retry-requests into e9e4170 on GoogleCloudPlatform:master.

[heidmotron]

@callmehiphop this works, thanks!

[callmehiphop]

Closing in favor of #1273