If the gtid_domain_pos of the master is ever modified,
gtid-variables will have multiple domains. Generally, we are
only interested in the most recent domain. This is tracked in
gtid_domain_id:s and the value of the master is used for
filtering the correct domain from all gtid-values.
Also, use gtid_current_pos instead of gtid_slave_pos. The
advantage of current_pos is that the same variable works also
for master servers. The gtid-handling is now more thorough and
detects some weird situations.
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