sstables: Switch index_list to chunked_vector to avoid large allocations
sstables: Switch index_list to chunked_vector to avoid large allocations

If there is a lot of partitions in the index page, index_list may grow
large and require large contiguous blocks of memory. That puts
pressure on the memory allocator, and if memory is fragmented, may not
be possible to satisfy without a lot of eviction.

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