Replace "master" and "slave" terms

Created by: ianbrayoni

Inspired by this change request

master and slave are a reminder of a painful world history, that is, they are racially charged terms.

I move that we purge these terms in favour of:

• primary, replica
• primary, secondary

Popular open-source communities have adopted newer references: https://github.com/django/django/pull/2692 https://github.com/antirez/redis/issues/3185 https://issues.apache.org/jira/browse/COUCHDB-2248