Due to migration, killSession and other implicit transaction abort, multi-statement transaction functions need to consider the concurrency even if the session has been checked out.

If a high txnNumber is received from migration, the current transaction needs to abort. (SERVER-33432)
If migration has the same txnNumber, the current transaction must be in state None (retryable writes).
All accesses to the transaction functions must check the txnNumber and the state at the beginning.