added missing closure for checkpointQueue in SqlLedger

[gabor-aranyossy]

The checkpointQueue in SqlLedger was not closed properly causing annoying errors like: ERROR c.d.p.s.stores.ledger.sql.SqlLedger - checkpoint queue has been closed with a failure! akka.stream.StreamDetachedException: Stream is terminated. Materialized value is detached. when stopping the Sandbox.

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[gabor-aranyossy]

@gerolf-da this was clearly an issue, but I doubt it's the actual cause for the perf-test timeouts in CI.