Update and extend AtomDBProxy client API

[andre-senna]

Update current add_atoms() command to be compliant with recent changes in AtomDB and add a new delete_atoms() method as in the AtomDB API:

    virtual vector<string> add_atoms(const vector<atoms::Atom*>& atoms,
                                     bool throw_if_exists = false,
                                     bool is_transactional = false) = 0;
    
    virtual uint delete_atoms(const vector<string>& handles, bool delete_link_targets = false) = 0;