Range queries for synchronization

[Scooletz]

Provide primitives needed for synchronization using Paprika. The most important one is a range query that can be executed over a readonly transaction. This query should provide:

1. ranges accounts by their hash,
2. ranges of storage items per account.

Probably the easiest implementation would be based on a cursor approach, which would allot to provide an IEnumerable or ref-based enumeration. The cursor implementation would be similar to LMBD stack approach, where for cursor serving they remember a vector over all the pages deep. The reason for that is that neither LMBD or Paprika implements B+ tree properly, meaning, that the pointer to the next page on the leaf level is missing. This has already been implemented in LMBD and we can follow the same path. So if you open a readonly batch for the given block, you can read as long as you need to providing a cursor like-behavior.

One could think of providing a next pointer in the page. This is against the COW design and breaks it terribly. It's discussed in depth in Howard Chu at this LMBD talk

[Scooletz]

This is needed for snap serve so not needed now.