We make CDC_GENERATIONS_V3 single-partition by adding the key
column and changing the clustering key from range_end to
(id, range_end). This is the first step to enabling the efficient
clearing of obsolete CDC generation data, which we need to prevent
Raft-topology snapshots from endlessly growing as we introduce new
generations over time. The next step is to change the type of the id
column to timeuuid. We do it in the following commits.
After making CDC_GENERATIONS_V3 single-partition, there is no easy
way of preserving the num_ranges column. As it is used only for
sanity checking, we remove it to simplify the implementation.
Update the `Generation switching` section: most of the existing
description landed in `Gossiper-based topology changes` subsection, and
a new subsection was added to describe Raft group 0 based topology
changes. Marked as WIP - we expect further development in this area
soon.
The existing gossiper-based description was also updated a bit.
