When describing a table, we need to do it carefully: if some
columns were dropped, we must specify that explicitly by
```
ALTER TABLE {table} DROP {column} USING TIMESTAMP ...
```
in the result of the DESCRIBE statement. Failing to do so
could lead to data resurrection.
However, if a table has been altered many, many times,
we might end up with a huge create statement. Constructing
it could, in turn, trigger an oversized allocation.
Some tests ran into that very problem in fact.
In this commit, we want to mitigate the problem: instead of
allocating a contiguous chunk of memory for the create
statement, we use `fragmented_ostringstream` and `managed_string`
to possibly keep data scattered in memory. It makes handling
`cql3::description` less convenient in the code, but since
the struct is pretty much immediately serialized after
creating it, it's a very good trade-off.
We provide a reproducer. It consistently passes with this commit,
while having about 50% chance of failure before it (based on my
own experiments). Playing with the parameters of the test
doesn't seem to improve that chance, so let's keep it as-is.
Fixesscylladb/scylladb#24018
ac9062644f