Use lock syncronize on transaction rollback (#73)

Avoids race conditions between counting the number of open transactions
and calling rollback.

Co-authored-by: Malcolm Locke <malc@riparide.com>

Author: malclocke

lib/database_cleaner/active_record/transaction.rb

@@ -19,8 +19,10 @@ def start
 
       def clean
         connection_class.connection_pool.connections.each do |connection|
-          next unless connection.open_transactions > 0
-          connection.rollback_transaction
+          connection.lock.synchronize do
+            next unless connection.open_transactions > 0
+            connection.rollback_transaction
+          end
         end
       end
     end