Flakiness: TestRedLock.test_locking_dogpile[memcached_cache]

Often getting this test failure in the CI (only with the memcached backend):
```
______________ TestRedLock.test_locking_dogpile[memcached_cache] _______________

self =
mocker =
cache = MemcachedCache (127.0.0.1:11211)

async def test_locking_dogpile(self, mocker, cache):
mocker.spy(cache, "get")
mocker.spy(cache, "set")
mocker.spy(cache, "_add")

async def dummy():
res = await cache.get(Keys.KEY)
if res is not None:
return res

async with RedLock(cache, Keys.KEY, lease=5):
res = await cache.get(Keys.KEY)
if res is not None:
return res
await asyncio.sleep(0.1)
await cache.set(Keys.KEY, "value")

await asyncio.gather(dummy(), dummy(), dummy(), dummy())
assert cache._add.call_count == 4
assert cache.get.call_count == 8
> assert cache.set.call_count == 1, cache.set.call_args_list
E AssertionError: [call(, 'value'), call(, 'value')]
E assert 2 == 1
E + where 2 = .call_count
E + where = MemcachedCache (127.0.0.1:11211).set

tests/acceptance/test_lock.py:49: AssertionError
```

It seems like the most likely cause to me is that on some race condition 2 calls to `client.add()` succeed, resulting in 2 coroutines entering the lock at the same time. In theory, only one call should be possible to succeed, the other should error because the key already exists. If this is caused by the memcached backend, then maybe we need to do something to work around it and provide a better guarantee that the lock works as expected.