Initial version really by Pawel, but many changes in between.
Big outstanding issues:
* span cache reclamation (unbounded memory otherwise...)
* bad block service detection and workarounds
* corrupted blocks detection and workaround
Co-authored-by: Paweł Dziepak <pawel.dziepak@xtxmarkets.com>
The main thing that's added is full RS support, but a lot of things
were rejigged along the way. The tests are still a bit lacking,
and will be augmented in future commits.
