MDEV-26: Global transaction ID.

Change of user interface to be more logical and more in line with expectations
to work similar to old-style replication.

User can now explicitly choose in CHANGE MASTER whether binlog position is
taken into account (master_gtid_pos=current_pos) or not (master_gtid_pos=
slave_pos) when slave connects to master.

@@gtid_pos is replaced by three separate variables @@gtid_slave_pos (can
be set by user, replicated GTIDs only), @@gtid_binlog_pos (read only), and
@@gtid_current_pos (a combination of the two, most recent GTID within each
domain). mysql.rpl_slave_state is renamed to mysql.gtid_slave_pos to match.

This fixes MDEV-4474.

sql/rpl_mi.h

@@ -130,10 +130,14 @@ class Master_info : public Slave_reporting_capability
   DYNAMIC_ARRAY ignore_server_ids;
   ulong master_id;
   /*
-    True if slave position is set using GTID state rather than old-style
-    file/offset binlog position.
+    Which kind of GTID position (if any) is used when connecting to master.
+
+    Note that you can not change the numeric values of these, they are used
+    in master.info.
   */
-  bool using_gtid;
+  enum {
+    USE_GTID_NO= 0, USE_GTID_CURRENT_POS= 1, USE_GTID_SLAVE_POS= 2
+  } using_gtid;
 };
 int init_master_info(Master_info* mi, const char* master_info_fname,
 		     const char* slave_info_fname,