Get your context value faster

Replace

v := ctx.Value(key)

With

v := goctx.Value(ctx, key)

There will be little difference when there is only 1~2 context.WithXXX calls

With 5 context.WithXXX calls

With 20 context.WithXXX calls

As we can see from the benchmark test, the map implementation is slower than the standard one, so it is not recommended to use RegisterValueFunc to register a context value function, unless you do not want to see nested stack with Value method call (That's also the reason why RegisterValueFunc is kept even it is not fast)