Memory/threads leak

Repeatedly calling etcd3.client(...) in a loop will leak lots of memory over time. The main reasons seems to be that this creates a watcher thread every time. This thread is never stopped or deleted thus causing memory to grow without bounds.

Start by reproducing repeated etcd3.client(...) calls in a loop and inspect the watcher thread lifecycle. Trace why each client-created watcher remains alive; done means repeated client creation no longer causes unbounded watcher threads or memory growth.

Written by the indexing model from the issue text.