Add support for statefiles to js_shared_dict_zone.

[jo-carter]

For persistently storing the state of the shared dictionary across stop & starts of nginx, and to support loading a pre-defined set of key-value pairs into a shared dictionary without hacks.

There are certain cases where nginx must be stopped & started to make configuration changes, such as when adjusting the limit_req_zone's key. This feature would preserve shared_dictionary's values in those cases.

This would work in a similar fashion to NGINX Plus's Keyval Zone.