Python: added support backend_id to route list.
Monitor: fixed initialization of backend_stat_provider.
Added new commands string presentations.
Python: removed group_id from elliptics.Address because now node's backend can be in different groups.
Updated elliptics.RouteList interface for working with multibackend nodes:
  * Replaced get_address_group_id() by get_address_groups()
  * Replaced addresses_with_id() by get_unique_routes() which returns routes unique by (address, group_id, backend_id) that can be used for routing to exact backend at address.
  * Added get_id_routes(id) - returns information which nodes (address, group_id, backend_id) are responsible for the id.
  * Replaced get_address_id()/get_address_eid by get_address_unique_routes() that returns one route per each node backend.
  * Changed percentages() - now each address contains per backend and total percentage of responsible id ranges.
  * Fixed test compatibility with new abilities.