Allow setting the voters and non-voters in a single operation. This
ensures that the configuration changes are done atomically.
In particular, we don't want to set voters and non-voters separately
because it could lead to inconsistencies or even the loss of quorum.
This change also partially reverts the commit 115005d, as we will only
need the convenience wrappers for removing the voters (not for adding
them).
Refs: scylladb/scylladb#18793
28d1aeb1fa