Add ngx_http_lua_ffi_shdict_store_when for set_when and safe_set_when

[hnakamur]

This pull request adds ngx_http_lua_ffi_shdict_store_when
which is used to define set_when and safe_set_when methods in ngx.shared.DICT.

These methods are useful for avoiding race conditions between successive calls of get and set.
You pass old_value as well as value to set_when, and it changes the value only when the value for the key is still the same as old_value.
Adding this method is suggested in the comment by @doujiang24.

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

[hnakamur]

Test with the following pull request branches passes at https://travis-ci.org/hnakamur/lua-nginx-module/builds/569371351

• Add shdict set_when and safe_set_when lua-resty-core#268
• Add shdict set_when, safe_set_when, and store_when stream-lua-nginx-module#171