During the Shard Merge project there's a distinction between two migration protocols: "multitenant migrations" and "shard merge". But soon we'll delete multitenant migrations, and all migrations will use the shard merge protocol. In anticipation, let's refer to "shard merge" less in the code. E.g., rename tenant_migration_shard_merge_util.h/cpp to something else, or move its code into tenant_migration_util.h/cpp. Find other places in the code where "shard merge" won't make sense to future readers after multitenant migrations are gone.