fromShard is a connection string – misleading. _recvStartChunk should get a ShardId rather than a connection string, then the MigrationDestinationManager can get connection strings from the ShardRegistry. There's a ScopedDbConnection conn object that shouldn't be used anymore: use the ShardRegistry and Shard.
Start actually using StartChunkCloneRequest::createFromCommand, which is currently used no where.
Generally clean up the MigrationDestinationManager – e.g., we have "_from = fromShard" local variable, but pass fromShard through functions anyway.