Benchmark fixes

Delay beginning most writes until we enter poll()

-Rename namespace to grpc_health->grpc to match spec
-Proper use of NOT_FOUND status code
-Improve testing
-Add source distribution to artifact build

Yay compatibility and predictability!

Before we patched the link command, now we just patch `spawn` as an
updatable catch-all solution to ARG_MAX limitations on bash for MSYS and
MinGW and friends.

- Echo commands
- Remove generated code before pod install
- Don’t “clean” before “build”

Pending:

- Tag BoringSSL repo.
- Tag gRPC repo.
- Build the plugin, zip it, and add it to the release.
- Push Cocoapods to trunk.