Making Rusage a type alias rather than a type definition allows to avoid
type conversions in several places.
Type aliases are supported since Go 1.9, go.mod states Go 1.11 to be the
minimum supported version.
Support for Go1.9 and appengine was removed in https://github.com/grpc/grpc-go/pull/3767. But it looks like we still need some way to do things differently for certain appengine builds.
ChannelZ is a debugging feature in grpc. It has not had appengine support; this commit
removes its support for 1.6, too. This change is because in a recent sys/unix commit
1.6 support was broken.
