Add locking so that a new round can't start before the previous one is finished

[AnotherKamila]

Currently, the rounds are completely independent, and thus if the timing is tight (see also #10 ), a new round can be triggered before the previous one finished, which will mess up the state on the webserver and thus the round won't run correctly.

One of these days somebody should solve it using https://docs.buildbot.net/current/manual/configuration/interlocks.html .