ledger-on-memory: Do not create iterators on a mutable log. (#12626)

We must copy the slice of the log that we are reading, to ensure that we
are not iterating over the log while we mutate it in a separate
transaction. (In other words, objects must not escape the `readLog`
function.)

This is most likely the cause of a flaky test run that we saw recently.

CHANGELOG_BEGIN
CHANGELOG_END

ledger/ledger-on-memory/src/main/scala/com/daml/ledger/on/memory/InMemoryLedgerReader.scala

@@ -33,7 +33,10 @@ class InMemoryLedgerReader(
               metrics.daml.ledger.log.read,
               state
                 .readLog(
-                  _.view.zipWithIndex.map(_.swap).slice(startExclusive + 1, endInclusive + 1)
+                  _.view.zipWithIndex
+                    .map(_.swap)
+                    .slice(startExclusive + 1, endInclusive + 1)
+                    .toVector // ensure we copy the results so we don't keep a reference to the log
                 )
                 .iterator,
             )