Use 'context' in generated code

[gyuho]

Replace golang.org/x/net/context with context in generated code:

• google.golang.org/grpc (Replace "golang.org/x/net/context" with "context" and stop supporting go1.6 grpc/grpc-go#1160)
• github.com/golang/protobuf (TODO Question: move to Go 1.7+ stdlib 'context' package? golang/protobuf#336)
• github.com/gogo/protobuf (TODO Any plan to switch Go standard package 'context' gogo/protobuf#282)
• github.com/grpc-ecosystem/grpc-gateway (TODO Move to stdlib context grpc-ecosystem/grpc-gateway#343)

Not a blocker for Go 1.9 migration, or 3.3 release.

c.f. #6174

[gyuho]

With Go type alias and 1.10, this is resolved.