Do not generate offline report

For some reason report server stops before this request
and tank hangs
Stop server's event loop after test
ZMQ rework in BFG plugin

Stop iteration when 'stop' ammo received
Stop ZMQ's eventloop when tank stops
Merge branch 'master' of https://github.com/yandex-load/yandex-tank
Merge pull request #122 from toshic/master

Tank hangs fixes