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.

mysql-test/r/connect.result

@@ -7,6 +7,7 @@ db
 event
 func
 general_log
+gtid_slave_pos
 help_category
 help_keyword
 help_relation
@@ -18,7 +19,6 @@ plugin
 proc
 procs_priv
 proxies_priv
-rpl_slave_state
 servers
 slow_log
 table_stats
@@ -45,6 +45,7 @@ db
 event
 func
 general_log
+gtid_slave_pos
 help_category
 help_keyword
 help_relation
@@ -56,7 +57,6 @@ plugin
 proc
 procs_priv
 proxies_priv
-rpl_slave_state
 servers
 slow_log
 table_stats
@@ -91,6 +91,7 @@ db
 event
 func
 general_log
+gtid_slave_pos
 help_category
 help_keyword
 help_relation
@@ -102,7 +103,6 @@ plugin
 proc
 procs_priv
 proxies_priv
-rpl_slave_state
 servers
 slow_log
 table_stats