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
also fix a discrepancy where cli option `--cache-size` is documented as
`0 disables the cache` in `codex/conf.nim`, but previously the value `0` would
result in a cache being constructed with default parameter values for
`CacheStore.new()`
Closes#180
