rdbms: make getMasterPos() ignore GTIDs outside of gtid_domain_id
- Filter out GTIDs with a domain that is not the one binlog events would be written to if the Database handle was given write queries. Likewise for the MariaDB server_id component.
- Also improve MySQL GTID support to better match that of MariaDB. This covers position retrieval, replication waiting, and ranges in GTIDs (which are almost always present).
- Make some MySQLMasterPos variables private by making use of accesors instead.
- Store the gtids array keyed by domain ID for convenience.
- Clean up dynamic call to static method.