Adds the ability to simply check whether a TCP socket accepts
connections to determine if it is healthy.  This is a light-weight -
though less comprehensive than scripting - method of checking network
service health.

The check parameter `tcp` should be set to the `address:port`
combination for the service to be tested.  Supports both IPv6 and IPv4,
in the case of a hostname that resolves to both, connections will be
attempted via both protocol versions, with the first successful
connection returning a successful check result.

Example check:

```json
{
  "check": {
    "id": "ssh",
    "name": "SSH (TCP)",
    "tcp": "example.com:22",
    "interval": "10s"
  }
}
```