Fix bug and further optimizations in availability distribution

- There was a bug that resulted in only getting one candidate per block
as the candidates were put into the hashmap with the relay block hash as
key. The solution for this is to use the candidate hash and the relay
block hash as key.
- We stored received/sent messages with the candidate hash and chunk
index as key. The candidate hash wasn't required in this case, as the
messages are already stored per candidate.