DNS Poll - max stable Cluster Size = max DNS Entry Response Count

### Steps to reproduce

- Configuration Used
```
config :libcluster,
debug: true,
topologies: [
dns: [
strategy: Cluster.Strategy.DNSPoll,
config: [
poll_interval: 10_000,
query: "appname.something",
node_basename: "some-container"
]
]
]
```
- Strategy Used
`Cluster.Strategy.DNSPoll`
- Errors/Incorrect Behaviour Encountered
`Maximum stable Cluster Size is the number of DNS results returned.`

### Description of issue

- What are the expected results?
DNS query, I would not expect nodes to be removed if not in the DNS response. I would expect to trust the disconnect if a node times out with `net_ticktime` and is not actively being removed. For example, if you have 15 nodes and DNS replies with 5 random node IPs, the cluster will become unstable.

- Is the documentation incorrect?
Documentation does not mention that nodes will be removed when no longer in DNS. It just says:
> this strategy will periodically poll DNS and connect all nodes it finds.

Should we introduce a config flag to turn off removing nodes?