Scala Core: add schema matching for unversioned instances

[alexanderdean]

See #147

We want to find the highest matching schema version for an unversioned instance, where:

• matching means that the instance passes validation against the given schema version
• highest means that no schema versions with higher numbers pass

[alexanderdean]

I think @fblundun has some ideas on a performant algorithm here - Fred could you share?

[alexanderdean]

@chuwy - should this ticket move to Iglu Scala Client?

[alexanderdean]

Migrated snowplow/iglu-scala-client#53

[chuwy]

I actually think it should belong to core. But It also depends on when do we need it. I don't have plans to migrate iglu client to core in near version. It can introduce breaking changes in SCE, so decided to be very careful on it.

[alexanderdean]

Okay, will keep both open for now!

[fblundun]

My original idea was to do a binary search for the last matching schema, instead of just checking starting at the most recent schema and working backwards. However I now don't think this is a good idea, because it assumes that for any two schemas A and B such that A is matching and B is not, A is higher than B. This is not necessarily true - in general I think it is only true given the additional assumption that A and B have the same model and revision.

I suppose we could use that information to reduce the number of schemas we check - e.g. if 1-0-9 isn't matching, we know that 1-0-8 and below also aren't matching. So for each model-revision pair, we need only check the latest addition.

[aldemirenes]

Migrated to snowplow/iglu-scala-core#28