We currently use std::deque<> for when we need large random-access containers,
but deque<> requires nr_items * sizeof(T) / 64 bytes of contiguous memory, which can
exceed our 256k fragmentation unit with large sstables. The new
container, which is a cross between deque and vector, has much lower
limitations.
Like deque, we allocate chunks of contiguous items, but they are
128k in size instead of 512. The last chunk can be smaller to avoid
allocating 128k for a really small vector.
