Allow ability to prevent changes to specific Master record fields during manual merge.

When allowing users to change the master record in a manual merge there are some fields on the master that we want to lockdown and prevent from being modified during the merge. Ideally this would be configurable from the Merge Rules page where we can already configure rules for individual fields. Need an enhancement to allow an additional option to flag a field as something like "Read-Only" or "Change Not Allowed" or "Uneditable".