Follow-up to #7917
The size of an cf::column_family_info is 224 bytes, so an std::vector that
contains one for each column family may be very large, causing allocations
of over 1MB.
Considering the vector is used only for iteration, it can be changed to
a non-contiguous list instead.
Signed-off-by: Wojciech Mitros <wojciech.mitros@scylladb.com>
Closes#7973