Add a new session type for job queues

[aburgel]

/domain @Betterment/test_track_core
/no-platform

This introduces a JobSession (along with a DelayedJob plugin that makes use of it) that manages the lifecycle and keeps a cache of visitors. This improves performance by eliminating duplicate identity lookups within a job.

This addresses some of #55, but it keeps the OfflineSession for unmanaged contexts, like the command line and for looking up visitors by visitor ID.

I renamed the existing Session model to WebSession for clarity, this is much of the diff. There's a tiny bit of overlap between JobSession and SessionVisitorRepository, but it was only a few lines, so I opted for duplication rather than refactoring.

[nanda-prbot]

Needs somebody from @Betterment/test_track_core to claim domain review

Use the shovel operator to claim, e.g.:

@myname << domain && platform

[aburgel]

I'm not sure why we had these here. We require delayed_job in the engine, so this seems unnecessary. I'll see if any apps blow up with this change before merging.

[jmileham]

Cool, if this doesn't cause any test leakage or other issues with the require statements you pulled << domain lgtm! Thanks so much!

[nanda-prbot]

Approved! 🎆 ⛳ 🎇

[aburgel]

if this doesn't cause any test leakage or other issues with the require statements you pulled

ran a couple of app test suites against this, and all looks good.