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"
}
}
```
Apparently the execution of `TestCheckTTL` could sometime take > 10ms
causing it to fail. This commit will eliminate the race condition for
most modern hardware.
Diptanu Choudhury