PartitionHandling: split during commit can leave the cache inconsistent after merge

Say we have a cluster ABCD; a transaction T was started on A, with B as the primary owner and C the backup owner. B and C both acknowledge the prepare, and the network splits into AB and CD right before A sends the commit command. Eventually A suspects C and D, but the commit still succeeds on B before C and D are suspected. And SuspectExceptions are ignored for commit commands, so the user won't see any error.

However, C will eventually suspect A and B. When the CD cache topology is installed, it will roll back transaction T. After the merge, both partitions are in degraded mode, so we assume that they both have the latest data and the key is never updated on C.

From C's point of view, this is very similar to ISPN-3421. The fix should also be similar, we could delay the transaction rollback on C until we get a confirmation from B that T was not committed there. Since B is inaccessible, it will eventually get a SuspectException and the CD cache topology, at which point the cache is in degraded mode and it can wait for a merge. On merge, it should check the status of the transaction on B again, and either commit or rollback based on what B did.

We also need to suspend the cleanup of completed transactions while the cache is in degraded mode, otherwise C might not find T on B after the merge.