replicaonly: add changes file

changes/next/replicaonly

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+Description:
+
+add 'replicaonly' config option that blocks all non-silent writes
+
+(Silent writes are those where the modseq is specified in the write,
+so the highestmodseq doesn't get increased - these are the sort done
+by sync_server)
+
+Config changes:
+
+the boolean config option `replicaonly` (default: false) can be set
+to mark a server as only being a replica.  This will stop calalarmd
+from doing anything, and also deny any non-silent writes.
+
+
+Upgrade instructions:
+
+No change required - you can keep running without setting this config
+option on replicas, and they will behave as before (in particular,
+you will still have to make sure not to run calalarmd on replicas).
+
+GitHub issue:
+
+none