in the worker, eager load all unprocessed users

if there's an error in one import, we don't want it to block new imports from others

app/import_worker.rb

···
     loop do
       @user_threads.delete_if { |t| !t.alive? }
 
-      if user = User.with_unfinished_import.where.not(id: @user_threads.map(&:user_id)).first
+      users = User.with_unfinished_import.where.not(id: @user_threads.map(&:user_id)).to_a
+
+      users.each do |user|
         collections = user.imports.unfinished.map(&:collection)
         thread = UserThread.new(user, collections, @verbose)
         @user_threads << thread