Ensure that struct applier is not deleted util its in-fly txns are finished

[Gerold103]

Currently the applier txn processing doesn't seem to keep the applier object alive until all its txns are fnished. That could be the reason why in replica_txn_wal_write_cb() the trigger must go replica->applier->txn_last_tm instead of directly using the pointer at the applier owning this txn.

Need to investigate, whether appliers live as long as their txns. If they don't - make them. Then make applier directly assign its txn_last_tm without struct replica participation.