replication: don't rollback qsync limbo wait on fiber cancel

During iproto graceful shutdown which is WIP we cancel all iproto
request in progress. This causes election_qsync_stress test failure.

We shutdown master on waiting transaction confirmation from quorum
(which is never exist in this test). Currently on shutdown we rollback
transaction in this state. So that when previous master is restarted
after electing new master we don't expect the rollback on previous
master.

Let's keep the transaction in limbo if fiber is cancelled as our
direction is to do only quorum rollbacks.

Part of #8423
Closes #9480

NO_DOC=bugfix