WL#7083 step 3.7. GTID_MODE settable: Make commands accessing GTID_MODE safe for concurrent SET GTID_MODE.

@sql/rpl_gtid.h
- Require that get_gtid_mode() is used with a lock. Either the new
lock gtid_mode_lock, or global_sid_lock.rdlock (or wrlock), or
LOCK_msr_map, or mysql_bin_log.get_log_lock must be held.

@sql/item_func.cc
- Update wait_for_executed_gtid_set to use lock when reading gtid_mode
- Update wait_for_sql_thread_after_gtids ot use lock when reading
gtid_mode.

@sql/mysqld.cc
- Update main to use global_sid_lock.rdlock when reading gtid_mode.

@sql/rpl_binlog_sender.cc
- Update Binlog_sender::init to use global_sid_lock.rdlock when reading
gtid_mode.

@sql/rpl_gtid_execution.cc
- Update gtid_reacquire_ownership_if_anonymous to use
global_sid_lock.rdlock when reading gtid_mode.

@sql/rpl_gtid_misc.cc
- Implement the lock assertions for get_gtid_mode.

@sql/rpl_gtid_state.cc
- Update generate_automatic_gtid to use global_sid_lock.rdlock when
reading gtid_mode.

@sql/rpl_mi.cc
- Update Master_info::read_info to use global_sid_lock.rdlock when
reading gtid_mode.

@sql/rpl_slave.cc
- Update IO thread initialization code to use global_sid_lock.rdlock
when reading gtid_mode.
- Fix a strange condition where positions in heartbeat events are
sanity-checked. This should not depend on gtid_mode.  Also remove
check for mi->get_master_log_name() != NULL; this is redundant, since
if it was NULL the previous memcmp would crash.
- Update IO thread queueing code to use global_sid_lock.rdlock when
reading gtid_mode.
- Update CHANGE MASTER to use lock_msr_map when reading gtid_mode.

@sql/sql_class.cc
- Call gtid_post_statement_checks regardless of gtid_mode.

@sql/sys_vars.cc
- Update check_slave_skip_counter to use global_sid_lock.rdlock when
reading gtid_mode.
- Update check_enforce_gtid_consistency to use global_sid_lock.rdlock
when reading gtid_mode.
- Update check_gtid_next_list to use global_sid_lock.rdlock when reading
gtid_mode.

@sql/transaction.cc
- Clear the thd->status & SERVER_STATUS_IN_TRANS bit regardless of
gtid_mode.