Double retractions in the transaction log

[grigoryk]

Some fun transactor behaviour:

• (given cardinality one, unique/identity :test/ident)
• first transact: {:test/ident "One"} - say, it gets assigned e=1
• then transact:

  [[:db/add 1 :test/ident "Two"][:db/retract 1 :test/ident "One"]]

• then observe transaction log:

  [[1 :test/ident "Two" 1]
  [1 :test/ident "One" 0]
  [1 :test/ident "One" 0]]

This is likely happening due to the way retractions are processed out of temp_search_results and into the log. We check for change in values, and synthesize a retraction datom, but we also actually have a retraction datom as well, so we end up with a duplicate retraction when there should have been just one.