(feature) Central state management

[thomaspoignant]

Requirements

GO Feature Flag is fully stateless and for some features it can cause some issues if you have more than 1 relay proxy running.

This is the case for the notifiers for example, if you have 2 relay-proxies the notifier will be called by each relay-proxy when you change the configuration of your flag in your flag configuration.

It will be great to have a way to synchronise some information in a central state management to have a context about all the relay-proxy.

• This system will work for the notifier but should work for other needs in terms of synchronisation.
• This should be optional to setup this system.