Erratic test (test_mix_states) was fixed.

Sometimes we were unable to stabilize weights with certain amount of requests,
'couse requests can never went to needed node. We send stabilizing requests only
to necessary node now.
Cross-instance shared statistics attribute was fixed.
Merge pull request #77 from kis8ya/fix/erratic-test_mix_states

Erratic test (test_mix_states) was fixed.