switch from returning a list of trefs to a "Search context"
also include a ctx, with ref counter increased (but without holding commit_lock — so InnoDB can modify the graph between calls)

Two approaches:

• put the bloom filter into the search context:
  • after initial search, it's reset
  • on following searches it's preserved — ensures the search walks away from the target
  • if InnoDB modifies the graph:
    • detect that in read_next with a "version" counter
    • switch to trx context
    • restart the search from the beginning, skip as many found as needed to catch up (how to ensure there're no duplicates — check old bloom filter?)
• do not preserve the bloom filter
  • simply reject vectors that are closer to the target than the closest start node (= closest last found)
  • or farthest prev last found
  • it allows to switch seamlessly to the trx context without restarting the search
• in all cases start nodes are results from the last iteration
• the chunk size need to be tuned. may be ef is a good value?

second approach works with MyISAM. InnoDB needs work still