stat: added per-backend command (requests/bytes and more) statistics
session: fixed zero-length write - we must return error and do not attempt to write data
session: long lines cleanup
stat: added success/error requests in per-backend command stats
stat: added success/error requests in per-backend command stats
stat: fixed input stat name for success command counter
stat: added rps/bps counters, use pointers to command stat structs to allow in-place update
node: added boost_system library to ldflags
stat: added Diff() comments
stat: dump the whole data string if json unmarshal failed
stat-json: updated json names to the latest elliptics version
session: groups are unsigned int actually, make them so
stat: fixed avalable disk size calculation. Print into log if monitoring is not enabled on given node
Merge branch 'v2.26' of github.com:bioothod/elliptics-go into bioothod-v2.26

Conflicts:
	elliptics/session.go
	elliptics/session.h
	elliptics/stat.go
Merge branch 'bioothod-v2.26' into 2.26