Allow application to control how configuration is reloaded

When application reloads the configuration on the fly it would be helpful to be able to control values that could really be updated without configuration restart.

So application may adjust it's internal state to the new configuration values.
At the same time, other changed values are ignored.

For example if application receives SIGHUP and sees number of workers and DB connection url changed it may adjust real number of workers and ignore changed DB url.

The proposal is to let application to control this on per option basis