Abstract the session default database

The default database can now be manipulated with a set of functions
exposed by the maxscale/protocol/mysql.h header. This removes the need to
handle the structures themselves in the modules and is a step towards
moving the dcb->data contents inside the session.

include/maxscale/protocol/mysql.h

@@ -485,4 +485,23 @@ mysql_server_cmd_t mxs_mysql_current_command(MXS_SESSION* session);
 void mysql_num_response_packets(GWBUF *buf, uint8_t cmd,
                                 int* npackets, size_t *nbytes);
 
+/**
+ * @brief Return current database of the session
+ *
+ * If no active database is in use, the database is an empty string.
+ *
+ * @param session Session to inspect
+ *
+ * @return The current database
+ */
+const char* mxs_mysql_get_current_db(MXS_SESSION* session);
+
+/**
+ * @brief Set the currently active database for a session
+ *
+ * @param session Session to modify
+ * @param db      The new database
+ */
+void mxs_mysql_set_current_db(MXS_SESSION* session, const char* db);
+
 MXS_END_DECLS