This patch adds additional [ECS
fields](https://www.elastic.co/guide/en/ecs/current/ecs-tls.html).

Sample output of the `tls.*` fields with this patch is below. Note the
somewhat strange nesting of data in `issuer` and `subject`. This is per
the ECS spec, but a bit awkward. We may want to break this data out into
the more specific ECS `x509` type in the future. For UI work we are likely
fine to parse this on the client and display the CN section in most
cases.

```json
{
  "version": "1.2",
  "version_protocol": "tls"
  "cipher": "ECDHE-RSA-AES-128-GCM-SHA256",
  "server": {
    "subject": "CN=r2.shared.global.fastly.net,O=Fastly\\, Inc.,L=San Francisco,ST=California,C=US",
    "hash": {
      "sha1": "b7b4b89ef0d0caf39d223736f0fdbb03c7b426f1",
      "sha256": "12b00d04db0db8caa302bfde043e88f95baceb91e86ac143e93830b4bbec726d"
    },
    "not_before": "2019-08-16T01:40:25.000Z",
    "not_after": "2019-08-16T01:40:25.000Z",
    "issuer": "CN=GlobalSign CloudSSL CA - SHA256 - G3,O=GlobalSign nv-sa,C=BE"
  },
  "certificate_not_valid_before": "2019-08-16T01:40:25.000Z",
  "certificate_not_valid_after": "2020-07-16T03:15:39.000Z",
  "established": true,
  "rtt": {
    "handshake": {
      "us": 42491
    }
  },
}
```

Work goes towards elastic/uptime#161