BUG/MEDIUM: master/cli: Properly pin the master CLI on thread 1 / group 1

BUG/MEDIUM: master/cli: Properly pin the master CLI on thread 1 / group 1

A previous fix was pushed for that (13fb7170be "BUG/MEDIUM: master/cli: Pin
the master CLI on the first thread of the group 1" ). Unfortunately, instead
of the master CLI, it is the sockpairs between the master and the workers
that were pinned to the first thread of the group 1. So the crash is still
there.

So, again, to fix the bug the master CLI is now pinned on the first thread
of the first group.

 patch should fix the issue #2259 and must be backported to 2.8.