Updates and fixes

Mostly fixes are described in commit message. This is the more detailed description of some changes:

### Recovery:
Recovery now will skip keys without data because such keys could not be written - in the future these keys should be removed while defragmentation. Recovery learned to export statistics in json format. Recovery tests has been extended by cases that checks merge and dc recovery with dump file.

### Python & tools:
dnet_balancer now can remotely update ids on nodes/backends if '-u/--update' arg is presented.