Adds support for the reap lock.

[slackpad]

This is the Consul side of hashicorp/consul-template#511. Unit tests look good as a regression - I will do some integrated testing in the morning, similar to what was done in the CT PR.

/cc @sethvargo

[slackpad]

Did some testing with Docker. With a Consul 0.6.1 build I was able to get the busy loop problem to come up. Adding in the fix for that in go-reap, I then tried to get the "stolen wait" to happen, and was able to do so for watches, checks, and consul exec. For watches I got [ERR] agent: Failed to invoke watch handler '/tmp/zombie': wait: no child processes, for checks I got 2016/01/13 06:05:39 [WARN] agent: Check 'zombie' is now critical, and for consul exec I got it to spuriously report an exit code of 1.

Moving over to a build with this change, things worked well, even with multiple pokes. I could see the normal child process reaping messages in the logs.

Here's the Dockerfile I used:

FROM ubuntu
COPY bin/consul /tmp
COPY foo.json /tmp
COPY zombie /tmp
CMD ["/tmp/consul", "agent", "-dev", "-config-file=/tmp/foo.json"]

And the consul foo.json:

{
        "log_level": "debug",
        "bind_addr": "0.0.0.0",
        "check": {
                "id": "zombie",
                "name": "make a zombie",
                "script": "/tmp/zombie",
                "interval": "10s"
        },
        "watches": [
                {
                        "type": "key",
                        "key": "foo/bar",
                        "handler": "/tmp/zombie"
                }
        ]
}

The zombie program is the same as hashicorp/consul-template#511.

To fire the watch:

date | curl -X PUT localhost:8500/v1/kv/foo/bar

And to remote exec:

./tmp/consul exec /tmp/zombie