perf(core): Use chain.from_iterable in threading.py (#614)
perf(core): Use chain.from_iterable in threading.py (#614)

This is a faster and more idiomatic way of using itertools.chain.
Instead of computing all the items in the iterable and storing them
in memory, they are computed one-by-one and never stored as a huge
list. This can save on both runtime and memory space.
python-zk/kazoo,python-zk/kazoo