This writer is used by streaming, on tablet migration and
load-and-stream.
The caller of distribute_reader_and_consume_on_shards(), which provides
a sharder, is supposed to ensure that effective_replication_map is kept
alive around it, in order for topology coordinator to wait for any writes
which may be in flight to reach their shards before tablet replica starts
another migration. This is already the case:
1) repair and load-and-stream keep the erm around writing.
2) tablet migration uses autorefreshing_sharder, so it does not, but
it keeps the topology_guard around the operation in the consumer,
which serves the same purpose.
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