Sql. Avoid using blocking api in sql threads

As for now, there are two places which blocks sql thread in order to wait for completion of the operation:

• join() on futures returned by UpdatableTable in ModifyNode
• finalisation of a transaction in QueryTransactionWrapper

Performance of a sql engine is sensitive to a blocking of sql threads because every fragment of a query is bound to a particular thread.

Let's revise and fix aforementioned places.