Change snapshot lookups

[pjaspers]

Seeing a lot fo these:

"ActiveRecord::StatementInvalid: PG::OutOfMemory: ERROR:  out of memory\nDETAIL:  Failed on request of size 39786433 in memory context \"printtup\".\n: SELECT  \"dhis2_snapshots\".* FROM \"dhis2_snapshots\" WHERE \"dhis2_snapshots\".\"project_anchor_id\" = $1 AND \"dhis2_snapshots\".\"id\" = $2 LIMIT $3"}

So as a theory, reworking the way we look up the nearest one. Keeping it more in postgres than in ruby. (Theory being that Rails is loading the snapshots with the '*' which is is causing postgres to load all the columns, including a large jsonb one, and by using a more selective approach we don't hit that)