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.
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.
