Before this patch, repair master and followers use their own schema
version at the point repair starts independently. The schemas can be
different due to schema change. Repair uses the schema to serialize
mutation_fragment and deserialize the mutation_fragment received from
peer nodes. Using different schema version to serialize and deserialize
cause undefined behaviour.
To fix, we use the schema the repair master decides for all the repair
nodes involved.
On top of this patch, we could do another step to make sure all nodes
has the latest schema. But let's do it in a separate patch.
Fixes: #4549
Backports: 3.1
3db136f81e