Adjust terminology: master/slave => leader/follower

[jsteemann]

The arangodb starter uses terminology in its log messages, documentation etc. that can be considered offensive.
The most prominent terms in use are "master" and "slave" here.

While these terms used to be widespread in the database and distributed systems sector, many products and projects have started moving away from them recently, trying to find replacement terms which are considered less offensive (in this particular case it could be "leader" and "follower").

Using the latest state of the "master" branch (ouch!):

$ grep -ri "\(slave\|master\)"  client/ docs/ examples/ service/ test tools *.go *.md | wc -l
820

I suggest to do a bulk change of these terms here when only internal variable names are affected. If renaming also affects command-line options etc., there should be migration path, if possible.

Thanks!