after a lot of discussion between @Fi3, @TheBlueMatt, @Sjors, @GitGab19 and myself, we realized that some aspects of the spec were not clear

in summary:

• "async" terminology is causing a lot of confusion... different people hold different interpretations of the term, so we need more clear wording around this feature
• ShortIds and SipHash were originally introduced as a strategy for optimizing bandwidth of DeclareMiningJob, but it was later realized that the computation trade-offs don't really seem to justify it (as discussed in Make more clear how we use the block header to calculate the txs short ids (job declarator) #99)
• DeclareMiningJob message needs a prev_hash field, so that JDS can avoid acknowledging work for old tips
• JDP SubmitSolution message needs a version field so that JDS can reconstruct the block before propagation (as it was partially rolled)
• remove extranonce_size, min_ntime and version fields of SetCustomMiningJob message are redundant and should be removed