Implement TTLs for records

This is an implementation of record TTLs (#5). This means for any record discodns can respond to, you can also provide a Time To Live value. This initial version isn't as optimal as it possibly could be (see the code comments) but it's a good first go.

Essentially, appending `.ttl` to any record key will to the trick. For example...

```
/net/disco/bar/.A -> 1.2.3.4
/net/disco/bar/.A.ttl -> 300
```

You can also add TTL values to records in subdirectories.

```
/net/disco/bar/.A/foo -> 1.2.3.4
/net/disco/bar/.A/foo.ttl -> 300
```

If a bad TTL value (i.e `-200`) is input into the ttl key, the record will still be returned, but with a TTL of zero. If debug mode is enabled, discodns will log these errors.