Reachability refactor

[coderofstuff]

Follow-up to #325

Reachability store should be refactored in the same way to alleviate the current bottleneck of storing reachability data in a one struct.

Context: performance of IDB is impacted when the DAG becomes wider and RelationsStoreExtensions::insert_batch can take a lot of time. This is due to the block children being stored under one key making the operation to add child blocks O(N^2).

Extra Caveat: in reachability store the children are sorted by their reachability interval to make binary search possible later. If each child is saved separately they won't be sorted. Solution: save the children unsroted, and sort them only when they are cached

[coderofstuff]

Fixed by #347