io: fixed buffered_device() constructor from std::string.
io: added lots of logs with URL string

When buffered_device is created, it moves string into its private location,
then it creates data pointer which only points to that content, not copies it.
When later we delete this device, its string content dissapears, while data pointers
still looks into that memory.

Copy string content into data pointer instead.

There is another bug in iodevice logic - we add device, call its ->read() method,
and in completion callback remove device from m_devices(), which frees device, while
we are still in its context.
buffer-device: get rid of internal string
io: do not request checksums for the second and the rest chunks. This is temporal change until elliptics backends start supporting partial checksums
io: added success/error groups debug into writing path. write command returns success/error groups in result json now too.
io: use json array when returning success/error write groups, cleaned up write logs
package: depend on 2.25.5.0+ elliptics-dev/elliptics-client
Return back bulk dnet_add_state()/add_remote()
This reverts previous revert of this commit, since there was no appropriate elliptics version.
stat: initial implementation of the internal stats fetching via /stat/ handler
stat: put backend monitor json object into node_stat. Final stat json contains monitor json as 'backend' object.
v2.25.0.10.1