watch_prefix memory leak

**memory leak of continually calling watch_prefix() when cancel() not called**
```
import etcd3
import tracemalloc

e = etcd3.client()

for i in range(100000):
e.put('/foo/bar', b'value')

tracemalloc.start()
while 1:
snap = tracemalloc.take_snapshot()
for s in snap.statistics('filename')[:1]:
print(s)
i = 0
it, c, = e.watch_prefix('/foo', start_revision=1)
for ev in it:
i += 1
if i > 1000:
break

Start by running the supplied tracemalloc reproduction around repeated watch_prefix('/foo', start_revision=1) calls, noting that the iterator is stopped after 1,000 events without cancel(). Trace the watch_prefix entry point and its iterator and cancellation handling; done means repeated calls no longer show unbounded memory growth when cancel() is omitted.

Written by the indexing model from the issue text.