With this feature it's possible to create parallel (via different attachments) processes reading consistent data from a database.
For example, a backup process may create multiple threads paralleling read data from the database.
Also a web service may dispatch distributed sub services paralleling doing some processing.
That is accomplished creating a transaction with SET TRANSACTION SNAPSHOT [ AT NUMBER <snapshot number> ] or isc_tpb_at_snapshot_number.
The <snapshot number> from the first transaction may be obtained with RDB$GET_CONTEXT('SYSTEM', 'SNAPSHOT_NUMBER') or transaction info call with fb_info_tra_snapshot_number.
Also added CORE-6017 - Add transaction info fb_info_tra_snapshot_number.
