Data tiering: fiber preemption and PreUpdate && PostUpdate might invalidate iterators

[kostasrim]

An example of this is OpMutableGet in string_family.cc:179. It first calls auto res = op_args.shard->db_slice().FindAndFetchMutable(op_args.db_cntx, key); which might preempt the fiber and interleave calls toPreUpdate && PostUpdate. Furthermore, iterators might be invalidated and a second Find is required. Also, investigate other commands as well.