Fix #397: Change master/slave to primary/replica

Created by: chriscoffee

This is somewhat of a tricky pull request that with all honesty I don't expect to get merged.

There are still some that feel that there are benifits of using the 'well-known' terminology of master and slave far rather than those without negative historic connotations such as leader/follower or primary/replica citing readability etc..

However, I hope you will atleast consider seriously altering the language in this repository, especially if it's helpful as a tool for interviews to work on large-scale systems given many companies have adopted either leader/follower or primary/replica, as alternate terms of reference.

Some examples its alteration, and conversation around it can be found below. I have tried to find some who haven't made the move in order to provide a balanced argument, feel free to add more in the comments:

• https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_ReadRepl.html

• https://docs.microsoft.com/en-us/previous-versions/system-center/system-center-2012-R2/hh846234(v=technet.10)

• https://github.com/django/django/pull/2694

• https://docs.pivotal.io/p-mysql/2-2/about-leader-follower.html

• https://www.ibm.com/support/knowledgecenter/SSSH5A_8.0.0/com.ibm.rational.clearquest.ms_admin.doc/topics/c_cr_db_replica_ovw.htm

• https://issues.apache.org/jira/browse/COUCHDB-2248

• https://support.cloud.engineyard.com/hc/en-us/articles/205408108-Set-Up-Database-Replication

• https://www.theregister.co.uk/2018/09/11/python_purges_master_and_slave_in_political_pogrom/

• https://github.com/antirez/redis/issues/3185

• https://www.drupal.org/project/drupal/issues/2275877

I think even if there's just a footnote referencing that some feel that this is no longer an acceptable reference, that's also an option.