feature: add a `unlink_from_stream(stream_name, ids)` method to the repository

[Bertg]

Today we already have a delete_stream option which allows us to remove and rebuild streams. This is pretty good, however if we are looking into rebuilding a stream where only a small part of the events should be removed then removing and rebuilding a stream is a big ask.

If I have a stream where a few events were wrongly assigned to, it would be ideal to be able to remove these easily. eg: unlink_from_stream("THING$123", ["a5ad1617-b3bb-42cb-b85f-d7aa71f26bd0"]).

I understand that there are some issues here:

What if there is position value? -> We could error when we try to delete an event from a stream that has a specific position. Possibly we can make a unsafe_unlink_from_stream("THING$123", ["a5ad1617-b3bb-42cb-b85f-d7aa71f26bd0"]) so also events with position can be deleted.

As developers we can already do this, by directly modifying the storage layer DELETE FROM event_store_events_in_streams WHERE event_id in ('a5ad1617-b3bb-42cb-b85f-d7aa71f26bd0') AND stream = 'THING$123' AND position IS NULL does the same thing.

But I believe that having it in the lib would be useful.

[mostlyobvious]

I agree in general than unlink would be useful counterpart to link.

Regarding unlinking events with a strict position (having integer valuer in position column in case of SQL adapter) — I'd check what are the guarantees that RES provides regarding monotonicity. Are the positions expected to be only monotonic or do they also have to be continuous? Is (1, 2, 6) a valid set of positions for a stream consisting of 3 events?

One place to verify is the aggregate root repository logic. If the logic counts stream events that's ok. Otherwise, relying on last known version could lead to mishaps. There might be lot's of 3rd party code making assumptions thus it could be a breaking change to introduce.

Another place to verify is the implementation of optimistic concurrency control. With continuity we can assume that concurrent writes will fail the uniqueness constraint on position. Without continuity, we'd have to rely on checking monotonicity instead.