core: Adds @CheckReturnValue to CallOptions

[groakley]

The handful of Google internal targets that violated this check have already been cleaned up. It looks like since @carl-mastrangelo first attempted this change, the errorprone tooling has improved such that this is no longer flagging false-positives, like unit tests.

Fixes #3671

[carl-mastrangelo]

LGTM

[ejona86]

Woo!

[dapengzhang0]

@groakley Do you happen to know which commit or CL in errorprone tooling improved such that this is no longer flagging false-positives, like unit tests? If open source users have a lot false-positives (although not likely), and they have to bump errorprone versions, that might break grpc-java build because @DoNotMock was removed from errorprone lately in open source world.

[groakley]

@dapengzhang0 I don't have a comprehensive list, but here are a couple of representative issues that were addressed (both early 2016):

• CheckReturnValue and ReturnValueIgnored should disable checking on mocked objects google/error-prone#226
• Disable @CheckReturnValue checking if the next statement is "fail();" google/error-prone#385

These sound like the false positives that @carl-mastrangelo was describing.

I would venture to guess v2.0.9 would be the first release to contain those changes