Batch etsource cache reads with fetch_multi to reduce N+1 queries

[aaccensi]

Context

Since adopting Solid Cache, several controller actions that trigger a GQL calculation are flagged by Sentry as N+1 queries. This was not visible before because Memcached round-trips are cheap and don't use SQL; with Solid Cache every cache operation is a SQL query. The fix implemented is cache-backend agnostic.

Implemented changes

MeritOrder collapses 6 individual cache fetches into one fetch_multi call. Molecules does the same for its 2 keys. Both use Thread.current to ensure fetch_multi runs at most once per request.

Notice that Fever and Reconciliation were not changed: Fever has a single key so batching adds no value, and Reconciliation has a dependency between its two keys (reconciliation_hash is computed using reconciliation_carriers, so they cannot be fetched in a single fetch_multi)

Related

Closes #1747

Checklist

• I have tested these changes (I believe this cannot be tester properly locally)
• I have updated documentation as needed
• I have tagged the relevant people for review