Show pipeline connection status

[sevein]

This pull request implements a basic pipeline status retrieval mechanism with caching. Definitely improvable.

From the web interface:

From the terminal:

$ curl -s "http://127.0.0.1:9000/pipeline?status=true" | jq
[
  {
    "id": "630fbda9-baa3-4464-9461-08bbe8b6359b",
    "name": "am",
    "capacity": 3,
    "current": 0,
    "status": "active"
  }
]

Fixes #519.