@sql/rpl_gtid.h, @sql/rpl_gtid_misc.cc
- Define the three allowed values for enforce-gtid-consistency.

@sql/sys_vars.cc, @sql/sys_vars.h
- Make enforce_gtid_consistency settable
- Create a new class for enforce_gtid_consistency
- Use global_sid_lock.wrlock when changing enforce_gtid_consistency.
- Define check_super_outside_trx_outside_sf also when HAVE_REPLICATION
  is not defined, since enforce_gtid_consistency now uses it.

@sql/binlog.cc:handle_gtid_consistency_violation
- Implement new logic for when a GTID-violation generates an
error/warning.

@sql/binlog.cc:is_ddl_gtid_compatible, @sql/binlog.cc:is_dml_gtid_compatible
- Use handle_gtid_consistency_violation to determine if an error/warning
should be generated.

@sql/binlog.cc:decide_logging_format,
@sql/rpl_gtid_execution.cc:gtid_pre_statement_checks
- Call is_[ddl|dml]_gtid_compatible regardless of
enforce_gtid_consistency.

@sql/share/errmsg-utf8.txt
- Update error messages to be consistent with new logic.