Rename stationId to checkerId

[juliangruber]

Since Station has been rebranded to Checker, rename stationId to checkerId

Tasks:

• Change Zinnia API
• Change zinniad env var
• Publish a new Zinnia version
• Update spark-checker to support both the old and new API
• Publish a new spark-checker version

[bajtos]

rename stationId to checkerId

How about using a more generic name like nodeId?

[juliangruber]

I was wondering about this as well! Thanks for bringing it up. Either works for me, but checker id is slightly preferred because we still make assumptions about the shape of the id, eg in

zinnia/runtime/runtime.rs

Line 176 in ad4eb49

static RE: Lazy<Regex> = Lazy::new(|| Regex::new(r"^[0-9a-fA-F]{88}$").unwrap());

so it's not really an implementation independent node id, and actually coupled to the checker node surrounding it.