Export grpc.Checker for compile-time interface checks of *SQLStorageAuthority, *SQLStorageAuthorityRO, and *RegistrationAuthorityImpl

[beautifulentropy]

Today we're enforcing that these implementations satisfy the grpc.checker interface conventionally, which places the onus on the reviewer. It would be much easier to have the compiler check this at compile time vis-a-vis var _ grpc.Checker = &RegistrationAuthorityImpl{}