Implement change validation for raft topology_change command. For now
the only check is that the "pending replicas" contains at most one
entry. The check mirrors similar one in `process_one_row` function.
If not passed, this prevents system.tablets from being updated with the
mutation(s) that will not be loaded later.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>