global-lock: add changes document

changes/next/global-lock

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+Description:
+
+Add a way to freeze an entire server temporarily while taking snapshots or
+similar.
+
+
+Config changes:
+
+Adds a new config switch `global_lock` - if true (the default) then a new
+global shared lock is taken before any exclusive lock is taken, and held
+until all global locks are released - meaning that any command which wishes
+to take a consistent snapshot can use the `cyr_withlock_run` command.
+
+Whether or not this setting is enabled, you can also use
+`cyr_withlock_run --user` to run a command with a single user locked.
+
+
+Upgrade instructions:
+
+There are no operational changes required; it just works once you're running
+the new version.
+
+
+GitHub issue:
+
+https://github.com/cyrusimap/cyrus-imapd/issues/1763