feat rocks: rework RocksDB bindings

[beryll1um]

• Use std::optional for Get methods instead of empty strings when NotFound
• Add RAII wrappers for RocksDB snapshots, iterators and write batches
  (rocks::Snapshot, rocks::Iterator, rocks::WriteBatch)
• Ensure graceful DB shutdown: sync WAL and wait for compaction/close on the
  dedicated Rocks task processor
• Link RocksDB privately from the rocks module (LINK_LIBRARIES_PRIVATE)

BREAKING CHANGES:

• storages::rocks::Client and storages::rocks::Component are removed
• Component is now components::Rocks with GetDb()
• Exception and RequestFailedException are removed in favor of StatusNokException
• Config keys changed: db-path/task-processor -> db_path/max_background_jobs

[beryll1um]

TODO:

• Better documentation
  (clarify architecture, object lifetimes, and RocksDB-specific caveats)
• Consider what configuration needs to be provided first
  (there is literally hundreds of different options across of interfaces in RocksDB)
• Tests for Snapshot, Iterator, and WriteBatch
  (cover consistency, iterator boundaries, and batch atomicity)
• Complete standard bidirectional iterator interface for Iterator class
  (account for limitations of RocksDB’s Next/Prev and seeking behavior)
• Improve finalization behavior
  (WAL sync and compaction may take significant time depending on write load and DB size)
• Sample for demonstration purposes
  (simple end-to-end example using snapshots, iteration, and batched writes)

...

[beryll1um]

I've started implementing a more or less viable iterator instead of directly binding RocksDB methods. It's still rough, and some parts (like the bool type cast) are just for testing, but the basic outlines are already in place.

I think it's better to make this pull a draft for now.

[beryll1um]

It looks like the Iterator is ready and needs to be reviewed. Unfortunately, implementing copying and postfix incrementing is impossible, as this would require copying, which is a rather complex operation in RocksDB (at least I didn't found a viable solution).