[evgeny-simplex]

In each notification delivery cycle, the server groups pending notifiers by the service of each notifier's queue in one pass, so the cycle cost is independent of the number of service records.

• memory queue store (QueueStore/STM.hs): the service is read from the queue record, and the loop over all service records is removed.
• PostgreSQL queue store (QueueStore/Postgres.hs): the notifier-to-service map is built from the same query with its two columns swapped, and the notifiers are grouped in one pass.
• tests: a grouping test for both stores in MsgStoreTests; newTestServiceRec is moved there from StoreLogTests unchanged.

The class method, its result type and the delivery loop are unchanged. Groups without notifiers are no longer returned by the memory store, and its groups are ordered by service ID, as in the PostgreSQL store.

/Core tests/Message store tests/
/Core tests/Store log tests/
/Notifications server