Fix bug introduced by missing renaming ttrss-session-id to ttrss-sid in some places
Implemetn nnttrss--{*} utility functions

* nnttrss--{read,write,update}-{headlines,article-map}
Refactor 'nnttrss--{read,write}-*' into 'nnttrss--{read,write}-vars'
First pass at implementing all required backend functions