change return type for `method getBlock` from `Future[?!(?Block)]` to
`Future[?!Block]`
use `type BlockNotFoundError = object of CodexError` to differentiate between
"block not found in the store" and other errors
also make some logic and error handling/messages more consistent across
BlockStore implementations
closes#177closes#182closes#210
alternative to #205, #209
BlockStore API got new return types (rationale in https://github.com/status-im/nim-codex/issues/123#issuecomment-1163797753):
- getBlock: Future[?! (?Block)]
- putBlock/delBlock/listBlocks: Future[?!void]
- hasBlock: Future[?!bool]
Plus refactored readOnce(StoreStream) and check received data in its tests.
And replaced local use of AsyncHeapQueue with seq.sort.
Michael Bradley, Jr
Bulat-Ziganshin