fs: fix failed token refresh on mounts created via the rc

Users have noticed that backends created via the rc have been failing
to refresh their tokens with this error:

    Token refresh failed try 1/5: context canceled

This is because the rc server cancels the context used to make the
backend when the request has finished. This same context is used to
refresh the token and the oauth library checks to see if the context
has been cancelled.

This patch creates a new context for the cached backends and copies
the global and filter config into the new context.

See: https://forum.rclone.org/t/google-drive-token-refresh-failed/22283

fs/filter/filter.go

@@ -615,6 +615,19 @@ func GetConfig(ctx context.Context) *Filter {
 	return c.(*Filter)
 }
 
+// CopyConfig copies the global config (if any) from srcCtx into
+// dstCtx returning the new context.
+func CopyConfig(dstCtx, srcCtx context.Context) context.Context {
+	if srcCtx == nil {
+		return dstCtx
+	}
+	c := srcCtx.Value(configContextKey)
+	if c == nil {
+		return dstCtx
+	}
+	return context.WithValue(dstCtx, configContextKey, c)
+}
+
 // AddConfig returns a mutable config structure based on a shallow
 // copy of that found in ctx and returns a new context with that added
 // to it.