Add health checks component

[oubiwann]

Tasks:

• [x] Create generic health check functions - see #18
• [x] Create core.async timer functionality - see #19
• [x] Create timer component - see #21
• [ ] Create cmr.dev.env.manager.components.dem.health
  • [ ] Ensure individual service state gets updated upon checks
• [ ] Create new component instance in cmr.dev.env.manager.components.system
• [ ] Add data structure for all desired / currently enabled (or supported) health checks in cmr.dev.env.manager.components.system
• [ ] Perform overall system checks, too
  • [ ] Ensure system state gets updated upon checks

Data structure might be something like this:

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

Depends on #21