CI documentation state change events

[tayloraswift]

goal: expose long poll API for getting notifications when documentation for a particular Edition is successfully uplinked

[tayloraswift]

alongside GraphStateLoop, which runs forever, we also need two additional immortal tasks, one for the server (which is currently the root task) and one for opening and iterating the MongoDB change stream cursor.

(sessions:Mongo.SessionPool) in 

GraphStateLoop.run
{
    (gsl:GraphStateLoop) in 

    async 
    let _:Void = forwardCursorItems(from: sessions, to: gsl, ...)
    //  Run forever...
    await ServerLoop.init(gsl, sessions, ...)
}

since we probably also want some sort of BuilderLoop following the same pattern, that’s going to involve nesting closures in closures, which might not scale. reminds me of withUnsafePointer pyramids…

[tayloraswift]

cc @Joannis

[tayloraswift]

i believe we eventually want to store the build outcome, even for failed builds on a per-version basis, to allow receiving notification of build failure as well.

since build outcome is accompanied by build logs, this means we should

• store log metadata in Snapshot instead of BuildMetadata

• add a task that regularly deletes logs older than 30d from the database and s3

• change s3 path computation to shard with a date prefix

[tayloraswift]

continuation in #237

[tayloraswift]

alternative, somewhat orthogonal feature implemented here: #238