CDC+LWT: fix missing CDC entries for successful LWTs
CDC+LWT: fix missing CDC entries for successful LWTs

Now, if CDC is enabled, `paxos_response_handler::learn_decision()`
augments the base table mutation. The differences in logic between:
(1) `mutate_internal<std::vector<mutation>>()`
and
(2) `mutate_internal<std::vector<std::tuple<paxos::proposal, schema_ptr, ...>>>()`
make it necessary to separate "CDC mutations" from "base mutation"
and send them, respectively, to (1) and (2).

Gleb explained in #5869 why it became necessary to add CDC code to LWT
writes specifically, instead of doing it somewhere central that affects
all writes:

"All paths that do write goes through mutate_internally() eventually so it
would have been best to do augmentations there, but cdc chose to log only
certain writes and not others (unlike MV that does not care how write
happened) and mutate_internal have no idea which is which so I do not have
other choice but code duplication. ... paxos_response_handler::learn_decision
is probably the place to add cdc augmentation."

Fixes #5869

scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla