Rename all instances of words master and slave

[IshaanDesai]

Main changes of this PR

Renaming all instances of words master and slave to primary and secondary or similar to this. Changes to names of variables are done according to the relevance of their use and the functionality of the class. This PR also renamer the words Master and Slave (with capital letters) in most places. The file MasterSlave.cpp and its use in other files is not changed as this would mask the changes done inside the file itself. The file name change is handled in: #1255

Motivation and additional information

We need to do this because we are in the 21st century ✊

Author's checklist

• I added a changelog file with make changelog if there are user-observable changes since the last release.
• I ran make format to ensure everything is formatted correctly.
• I sticked to C++14 features.
• I sticked to CMake version 3.10.
• I squashed / am about to squash all commits that should be seen as one.

Reviewers' checklist

• Does the changelog entry make sense? Is it formatted correctly?
• Do you understand the code changes?

[uekerman]

Still some work to do here 😁
Simple brute-force replacements do not work here as IMO "pimary" and "secondary" should only be used as adjectives (at least in comments).

Some common suggestions:

• "to the primary ..." -> "to the primary rank ..."
• "secondaries" -> "secondary ranks"
• "primary-primary com" -> "primary com"
• "master-slave mode": as the server mode does no longer exist, we can just drop these
• "on all slaves" or similar -> "on all ranks" as typically this also happens on the (previous) master rank

I hope I caught all occasions. See also the suggestions here: #1026 (comment)

In which order do you plan to merge this PR and #1255 ?
You will get nasty merge conflicts. It could be better start #1255 from scratch again.

[uekerman]

Did not go through everything again. Still open TODOs. Sorry 😁
I would advice against any "find and replace" here. Then, we end up with names like acceptSecondaryRanksConnection where acceptSecondaryConnections feels much more natural.

[uekerman]

Did look through the details again, but I have a good feeling.