Stepdown thread should perform w:all write after stepping up and restarting node

After the stepdown thread steps up a node, any of the other nodes in the set may go into rollback. This rollback can happen at unpredictable times, since it is based on when the node gets a new batch and triggers the OplogStartMissing error. When rollback occurs, the node closes all of its connections, and the stepdown thread is not robust to connections closing at arbitrary points. The stepdown thread can wait for all rollbacks to complete by performing a w:"all" write after stepping up a node.