Introducing DeltaReader

[citizen-stig]

This PR adds a DeltaReader, that will be used by new storage manager.

DeltaReader resembles combination of CacheDb and CacheContainer

It has public API that can be used by StateDb, AccessoryDb and LedgerDb the almost same way as CacheDb (excluding writes, which is usage is in process of being removed from SDK).

And it also manages ordering of keys and iteration for snapshots and database. It doesn't have any implicit link to StorageManager. It explicitly gets list of snapshots and DB and it uses it to provide correct data.

Current code has some duplication with existing CacheDb and CacheContainer. This is intentional. After SDK migrates to new StorageManager, old Cache* related structs will be removed.

Here is a list of all public methods of DeltaReader and where they have been copied (*means with significant modifications). new is ignored as it is obvious constructor.

• get : CacheDb::read + CacheDb::read_async. Now it does not lock, as it just iterate over Vec
• get_largest: CacheDb::get_largest. But simplified as it doesn't have to join "local_cache" into picture
• get_prev: CacheDb::get_prev. Same as get_largest
• get_n_from_first_match: CacheDb::get_n_from_first_match
• collect_in_range: CacheDb::collect_in_range

All iteration related logic is taken mainly from CacheContainer