There is no need to depend on the old context package now that the
minimum Go version is 1.7. The move to "context" eliminates our weird
vendoring setup. Some vendored code still uses golang.org/x/net/context
and it is now vendored in the normal way.
This change triggered new vet checks around context.WithTimeout which
didn't fire with golang.org/x/net/context.
Context keys must have a unique type in order to prevent
any unintented clashes. The code used int(1) as key.
Fix it by implementing the pattern recommended by package context.
