Add service status change time field

It would be nice to know when the last state transition for a service occurred by querying the check API, for example:

```
{
"service:mysrv": {
"Node": "srv1",
"CheckID": "service:mysrv",
"Name": "Service 'mysrv' check",
"Status": "passing",
"LastChangeTime": 123456789,
"Notes": "",
"Output": "",
"ServiceID": "mysrv",
"ServiceName": "mysrv"
}
}
```

UTC ISO8601 timestamp is slightly more readable, but a UNIX timestamp would be fine as well.