Add shdict set_when and safe_set_when

[hnakamur]

This pull request adds set_when and safe_set_when methods to ngx.shared.DICT.

The following two pull requests must be merged before merging this pull request.

• Add ngx_http_lua_ffi_shdict_store_when for set_when and safe_set_when by hnakamur · Pull Request #1574 · openresty/lua-nginx-module
• Add shdict set_when, safe_set_when, and store_when by hnakamur · Pull Request #171 · openresty/stream-lua-nginx-module

I hereby granted the copyright of the changes in this pull request to the authors of this lua-resty-core project.

[hnakamur]

Test with the following pull request branches passes at https://travis-ci.org/hnakamur/lua-resty-core/builds/569553742

• https://github.com/openresty/lua-nginx-module/pull/1574
• https://github.com/openresty/stream-lua-nginx-module/pull/171