Add this invariant back (It was removed byDecided that it is better not to add back the invariant so that it eases the updgrade/downgrade logic in case we ever want to support multiple concurrent chunk donations. SERVER-62245). This is possible after 6.0 because: (1) SERVER-60834and SERVER-62281provide a barrier on FCV upgrade to 6.0 that there are no unfinished migrations that possibly started with a binary affected by SERVER-62245; (2) After SERVER-62296, a second config.migrationCoordinators document won't possibly be inserted if there already existed one.
- No longer catch and ignore TransactionTooOld here and here. Also get rid of MigrationCoordinator::_recoveringMigration, since it is no longer needed.
- Delete jstests/sharding/recover_multiple_migrations_on_stepup.js