Read globally consistent snapshot of ACID tables

For ACID tables Impala fetches the valid write ids outside of a transaction. This means that when a query refers to multiple ACID tables (e.g. doing a join) the query might observe the modifications of the tables out of order.

E.g. let's suppose that the modification order is "write to A", then "write to B", but Impala might see "write to B" without seeing "write to A".

Instead of that Impala should open a transaction and fetch the valid write ids in the context of the opened transaction. This way Impala would see a consistent global snapshot of all the tables.