fix executed_gtid_set missing from instances without binlogs

[fuyar]

Fixes #803

Executed_Gtid_Set should be fetched for any MySQL instance and should not be restricted for "masters" (with binlog ON).

[shlomi-noach]

looks good, thank you! I'll send to testing and then merge.

[shlomi-noach]

This works correctly.
The downside is that we waste network traffic: show master status returns Executed_Gtid_Set anyway, and we ignore it where we do have binary logs. Executed_Gtid_Set may be a large (long) value for clusters suffering from multiple failovers.

Still, this is more correct to use. I'll merge it as-is, and consider a future optimization that only reads @@global.gtid_executed if no binary logs are available.