Fix possible deadlock when locking key files
Replace per zone structure mutex with a single mutex per zone name that locks key files for the zone in all views (instead of having a separate lock for each zone per view).
The fix was merged as part of #1126 (closed) in commit 28c51799, this MR now only deals with the CHANGES and Release Notes.
Closes #2786 (closed)
Edited by Matthijs Mekking