do_query() is a coroutine but uses some continuations to take
advantage of exceptions being propagated via future::then() without
being thrown. We can accomplish the same thing with a nested coroutine
and coroutine::try_future(), simplifying the code.
While this area isn't performance intensive, we're not adding allocations.
The coroutine frame may add an allocation, but since read_page()
certainly does not return immediately, the following then() will allocate
as well. Since we eliminated that then(), the change is at least neutral
allocation-wise.
Closesscylladb/scylladb#28258
874322f95e