style: migrate from slave/master terms to primary/replica

[imnotjames]

this change updates terminology to "primary"/"replica"

more inclusive terminology can lead to a more inclusive project

the terminology master/slave is considered non-inclusive by many
and most databases that this project uses are also migrating away
from the terminology.

this leaves the original "master" and "slave" terminology in place
but marks them as deprecated & points them at the primary/replica
terminology. at the next breaking change branch these can be removed
with a warning about the breaking change in the changelog

[imnotjames]

I did my best to ensure this wouldn't break existing code that may rely on the previous termonology.

However, if we wanted to make this into a breaking change that could be arranged. :)

[pleerock]

Unfortunately this is a breaking change, and we can't do it right now because it will break user code. But in the future versions, I'm sure we'll apply these changes.

[imnotjames]

This is a breaking change? What can we do to make it less of a breaking change?

I did what I could to ensure the changes wouldn't cause backwards compat breakages, thought it had worked per tests.

[imnotjames]

Either way, though, thanks for looking at it! Looking forward to when this change can be made. Happy to help out with it against whatever branch is the next breaking release. :)

[imnotjames]

I'll close this for now so we don't clutter up the pull requests with things that aren't gonna be merged in right away.