CassandraPersistenceQueries.currentEventsBefore can skip deleted partitions

[xirc]

For #201

CassandraPersistenceQueries.currentEventsBefore iterates events on all partitions (from + 1 to 0) but can skip deleted partitions by using the highest deleted sequence number (callled deleted_to), which reduces some read-events queries.

CassandraPersistenceQueries.currentEventsBefore:

akka-entity-replication/rollback-tool-cassandra/src/main/scala/lerna/akka/entityreplication/rollback/cassandra/CassandraPersistenceQueries.scala

Lines 177 to 189 in 1afe799

	override def currentEventsBefore(
	persistenceId: String,
	from: SequenceNr,
	): Source[PersistenceQueries.TaggedEventEnvelope, NotUsed] = {
	val fromPartitionNr = PartitionNr.fromSequenceNr(from, journalSettings.targetPartitionSize)
	val sources =
	for (partitionNr <- fromPartitionNr.value + 1 to 0 by -1) yield {
	currentEventsBeforeOnPartition(persistenceId, from, PartitionNr(partitionNr))
	}
	sources
	.fold(Source.empty)(_.concat(_))
	.mapMaterializedValue(_ => NotUsed)
	}

CassandraPersistenceQueries can fetch the highest deleted sequence number (called deleted_to) from the metadata table (akka.metadata).

deleted_to is described as:

• https://doc.akka.io/docs/akka-persistence-cassandra/1.0.5/journal.html#schema
• https://github.com/akka/akka-persistence-cassandra/blob/v1.0.5/core/src/main/scala/akka/persistence/cassandra/journal/CassandraJournal.scala#L526-L628