Streaming was keeping effective_replication_map_ptr around the whole
process, which blocks topology change barriers.
This will inhibit progress of tablet load balancer or concurrent
migrations, resulting in worse performance.
Fix by switching to the most recent erm on sharder
calls. multishard_writer calls shard_of() for each new partition.
A better way would be to switch immediately when topology version
changes, but this is left for later.
9dac0febce