v0.7.2

Quadratic growth of threads number has stopped. The problem was observer on Python 3.6 and 3.7 and was not on 3.8, because starting with 3.8 ThreadPoolExecutor does not spawn new thread if there are idle threads in the pool already. The issue was in the fact that for each of worker thread we run an event loop which default executor is the ThreadPoolExecutor with default (by Python) number of threads. All this eventually ended up in hundreds of thread created for each GraphqlWsConsumer subclass.