Support TCP checks

[armon]

For high volume HTTP checks, we can avoid fork/exec and do the TCP check in process

[armon]

On second thought, its better to push this externally. Reduces complexity.

[blalor]

Can you clarify?

[armon]

I think its simple enough to rely on standard unix commands like nc instead of building that into the core.

[mitchellh]

And to perhaps address the original issue, we've been bouncing ideas for long-running processes to avoid the fork/exec cost for thousands of services. Is this what you were thinking @armon?

[armon]

Yeah #132 is open for that exactly

[josephholsten]

Seems like the push-style checks are provided by the API endpoints /v1/agent/check/{status}/{checkId}. And hey, HTTP is TCP! Is there anything else required for this?