Use templates instead of compile params for mutex in hashmap

Change lock implementation in hashmap to use templates instead of
compile time parameter.

Changes are related to pretty complicated c++ syntax for
template derived class using a member it inherits from its template base class.
Coding convention is:
* To get types defined in base class in derived class
        using name_of_type = typename base_class::name_of_type
* To get access to methods defined in base class
        using base_class:method_name
* To get access to fields defined in base class
        this->field_name

As hash_map_node_base is now template, it's impossible to determine type
of empty_bucket defined in way it was until now - so now it's removed.
IMHO checking if node is nullptr is cleaner than comparing it to
empty_bucket(which is nullptr anyway)