Migrate from master/slave language

[bopopescu]

For diversity reasons, it would be nice to try to avoid 'master' and 'slave' terminology in this repository which can be associated to slavery. The master-slave terminology could be problematic for people in several countries which has the history of slavery like Romania, USA and many others. Thank you for considering the proposal. Let me know if any changes in the PR are needed, I would be happy to implement them.

[daniel-nichter]

Thanks for the PR @bopopescu. There are two issues that need to be addressed before we can merge this. First, MySQL's new official terminology is "source" and "replica":

• https://mysqlhighavailability.com/mysql-terminology-updates/
• https://dev.mysql.com/doc/refman/8.0/en/replication.html

Second, vars in the code that map directly to vars in MySQL output which haven't changed yet (e.g. Exec_Master_Log_Pos) need to remain the same else the code won't work.

If you can make these two changes, we'd be happy to merge this PR.

(Also re https://github.com/square/inspect/blob/master/CONTRIBUTING.md, have you signed the CLA?)