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limbo: make txn_limbo_confirm nop during fencing

Before this patch a synchronous transaction could be confirmed when user
lowered `replication_synchro_quorum` while limbo is in the frozen state.
It could lead to the next situation:
1) server1 loses a quorum of connections and its limbo goes into the
   frozen state.
2) The new elected master rollbacks a non-confirmed synchronous
   transaction.
3) On the old master we lower `replication_synchro_quorum` to 1. Our
   synchronous transaction is confirmed because our node can collect
   sufficient number of acks.
As a result, we have inconsistent state in the replicaset because on the
old master we have a confirmed transaction but on the new master we have
a rolledbacked transaction.

Now, we fix this issue by returning from the `txn_limbo_confirm` when
`txn_limbo_is_frozen` is true. It can help us to make the
`txn_limbo_confirm` - nop and not confirm synchronous transactions.

Needed for tarantool#11574

NO_DOC=bugfix
NO_CHANGELOG=<not released>

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