Context-aware methods to ProviderClient and ServiceClient

Provide new methods to leverage per-call `context.Context`.

This version, suitable to be backported to `v1`, vendors the context
package from the v1.21 standard library, to make "context.AfterFunc"
available.

context.go

@@ -0,0 +1,54 @@
+package gophercloud
+
+import (
+	"context"
+	"time"
+
+	future_context "github.com/gophercloud/gophercloud/internal/context"
+)
+
+type mergeContext struct {
+	context.Context
+	ctx2 context.Context
+}
+
+// MergeContext returns a context that is cancelled when at least one of the
+// parents is cancelled. The returned context also returns the values of ctx1,
+// or ctx2 if nil.
+func MergeContext(ctx1, ctx2 context.Context) (context.Context, context.CancelFunc) {
+	ctx, cancel := future_context.WithCancelCause(ctx1)
+	stop := future_context.AfterFunc(ctx2, func() {
+		cancel(future_context.Cause(ctx2))
+	})
+
+	return &mergeContext{
+			Context: ctx,
+			ctx2:    ctx2,
+		}, func() {
+			stop()
+			cancel(context.Canceled)
+		}
+}
+
+// Value returns ctx2's value if ctx's is nil.
+func (ctx *mergeContext) Value(key interface{}) interface{} {
+	if v := ctx.Context.Value(key); v != nil {
+		return v
+	}
+	return ctx.ctx2.Value(key)
+}
+
+// Deadline returns the earlier deadline of the two parents of ctx.
+func (ctx *mergeContext) Deadline() (time.Time, bool) {
+	if d1, ok := ctx.Context.Deadline(); ok {
+		if d2, ok := ctx.ctx2.Deadline(); ok {
+			if d1.Before(d2) {
+				return d1, true
+			} else {
+				return d2, true
+			}
+		}
+		return d1, ok
+	}
+	return ctx.ctx2.Deadline()
+}