Merge branch 'xlog_ng' into unstable

* xlog_ng: (22 commits)
  [feeder] support for tagged rows
  [core] fix bug: cookie is 64 bit
  [core] default tags are negative
  [core, box] row dispatch based on tag
  [core, box] wal_write and snapshot_write row now explicitly require tag and cookie
  [core, box] support for cookie in WAL
  [core] refactor read_u{8,16,32,4}
  [core] better error handling on remote recovery
  [core] add missing exception handling when recovering WAL
  [core] flush when saving snapshot
  [core] cleanup logflushing logic
  [core] refactor io rate limiting
  [core] remove dead code
  [core, box] support for configurable behavior on recovery errors
  [core, box] move snap_io_rate_limit to module specific initialization
  [core] use buffered read when doing recovery
  [core] fix packet length calculation
  [core] close connection to feeder in case of error
  [box] now replica reports its lag
  [core] hide v04 stuff
  ...