Create generic health check functions

[oubiwann]

For every type of supported health check, we will need a function to perform the check. So far, these are:

• [x] :tcp - a simple TCP "ping-like" functionality (e.g., TCP CONNECT-only) - connect to host and port
• [x] :http - an HTTP health check - do an HTTP GET against a specified endpoint and parse results
• [x] :docker - check the status of a running Docker container
• [x] :cpu - check the CPU usage of a process-based service
• [x] :mem - check the memory usage of a process-based service

Ultimately, these would be used like so at the system-component-level:

[{:name :mock-echo :type :tcp :frequency 30000}
 {:name :mock-echo :type :http :frequency 5000}
 {:name :elasticsearch :type :docker :frequency 10000}

But directly, these functions would probably be utilized by a new health method supported by service components -- those would be responsible for setting/configuring the proper hosts, ports, endpoints, containers, etc.

As such, dev tasks might be this:

• [x] Create generic TCP ping function ... maybe return {:connects? <bool>}
• [x] Create generic HTTP GETter with response body-parsing and HTTP status code-checking ... maybe return {:status <int> :value <parsed body>}
• [x] Create generic Docker container checker ... maybe return {:container-up? <bool>}
• [x] Create a generic process CPU checker ... maybe return {:cpu <float>}
• [x] Update ProcessRunner component to support a health method
• [x] Update DockerRunner component to support a health method
• [x] Implement health methods for each service type to call the appropriate generic health functions, dispatched on type

[oubiwann]

HTTP GETer done in b79c041.

[oubiwann]

Docker checker done in 06d8eca.

[oubiwann]

Added TCP ping support in 261f564.