Usually, a reconcilable_result holds very few partitions (1 is common),
since the page size is limited by 1MB. But if we have paging disabled or
if we are reconciling a range full of tombstones, we may see many more.
This can cause large allocations.
Change to chunked_vector to prevent those large allocations, as they
can be quite expensive.
Fixes#4780.
(cherry picked from commit 093d2cd7e5)
27de1bb8e6