Expiry strategy improvement request for DefaultDnsCache
### Expected behavior
Hostname configured with both IPv4 and IPv6 dns records. Netty dns resolver is configured to prefer IPv4 addresses, so I should always get IPv4 address.

### Actual behavior
but after several minutes run, it can only return ipv6 record.

### Steps to reproduce
Create a NettyDns resolver with DefaultDnsCache, and keep it running for several minutes. You can see finally only ipv6 record can be got.


### Minimal yet complete reproducer code (or URL to code)
the url have the issue is "mirrors.servercentral.com"

### Netty version
4.1.15.Final

### JVM version (e.g. `java -version`)
```
java -version
java version "1.8.0_131"
Java(TM) SE Runtime Environment (build 1.8.0_131-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.131-b11, mixed mode)
```
### OS version (e.g. `uname -a`)
```
uname -a
Darwin zhangjianyuans-MacBook-Pro.local 16.7.0 Darwin Kernel Version 16.7.0: Thu Jun 15 17:36:27 PDT 2017; root:xnu-3789.70.16~2/RELEASE_X86_64 x86_64
```
