Fixed two bugs of which one was older.
1. in query_classifier.cc autocommit_enabled, and transaction_active variables maintained their values across different sessions. Now those values are stored in each router_client_ses object.
2. As a part of implementation of MAX-95 session variables were added to BACKEND struct which is shared with all sessions using the SERVICE which the particular BACKEND serves. Now each router_client_ses object has a backend reference struct which includes pointer to BACKEND, DCB and to session command cursor.

Added test - set_autocommit_disabled.sql, test_after_autocommit_disabled.sql - to check that session variable is discarded when session where it belongs terminates.