In get_sstables_for_key in api/column_family.cc a set of lw_shared_ptrs
to sstables is passes to reducer of map_reduce0. Reducer then accesses
these shared pointers. As reducer is invoked on the same shard
map_reduce0 is called, we have an illegal access to shared pointer
on non-owner cpu.
A set of shared pointers to sstables is trasnsformed in map function,
which is guaranteed to be invoked on a shard associated with the service.
Fixes: #14515.
Closes#14532
61dc98b276