Flat storage post launch tasks

[Longarithm]

Plan for work after #7327 is finished:

1. Optimized read-only FS (TODO)

   • inline small values Implement inlining small values for FlatState #8243
   • estimate gas costs for inlined values
   • change gas costs to make small values cheaper
   • Quick flat state removal for non-tracked shard #8332, Add DB operation for deleting range of keys #8200

2. Extend FS to work for writes (DESIGN PHASE)

   • decide how we are going to do it Flat Storage for writes #8245
   • do as decided in previous point, specific work items TBD

[Longarithm]

Some items were completed, others are won't do because MemTrie is going to serve state reads&writes on execution path.