protoc-gen-go: use standard library context (requires Go1.9)

[dsnet]

This change effectively makes Go1.9 the minimum required version of Go
when using gRPC since type aliases are needed to truly unify context.
Even though this only affects those who use gRPC, there are other reasons we
need Go1.9 (such as type aliases for import public).

Fixes #537

[dsnet]

Even though AppEngine classic is still not on Go1.9, it should happen by the time we merge dev into master (#479).

[neild]

This doesn't make Go 1.9 the minimum unless you need to mix code which uses "golang.org/x/net/context" and "context". If you consistently use one or the other, there's no need for an alias.

[dfawley]

This doesn't make Go 1.9 the minimum unless you need to mix code which uses
"golang.org/x/net/context" and "context". If you consistently use one or the
other, there's no need for an alias.

gRPC-Go imports x/net/context, so yes, this does make Go 1.9 the minimum version supportable by gRPC-Go. This is not OK for us.