Random crashes if you commit active transaction within cursor block

[pbrumm]

I am putting this in as a warning for now. I will try to get a scenario that reproduces

row = nil
db.cursor do |cursor|
  begin
    while row = cursor.next
       # make some changes
    end
  ensure
    env.active_txn.try(:commit)
  end
end

something about how the cursor is released within the cursor will cause a random crash

row = nil

begin
  db.cursor do |cursor|
    while row = cursor.next
       # make some changes
    end
  end
ensure
  env.active_txn.try(:commit)
end

this code above doesn't crash

it appears there are protections in to handle this situation and it does work 90% of the time. Our main way of reproducing was to run a large test suite that is hitting a lot of lmdb data loading.