cdc::topology_description describes a mapping of tokens to CDC streams.
The cdc::generate_topology_description function is given:
1. a set of tokens which split the token ring into token ranges (vnodes),
2. information on how each token range is distributed among its owning
node's shards
and tries to generate a set of CDC stream identifiers such that for each
shard and vnode pair there exists a stream whose token falls into this
vnode and is owned by this shard.
It then builds a cdc::topology_description which maps tokens to these
found stream identifiers, such that if token T is owned by shard S in
vnode V, it gets mapped to the stream identifier generated for (S, V).
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