Support read-only tree nodes or some kind of access control

[dumbbell]

Why

In khepri_mnesia_migration, we use the ability to switch Mnesia tables to read-only to prevent further updates at the end of a copy from Mnesia to Khepri.

It would be nice to have a similar mechanism to be able to implement the opposite direction: a copy from Khepri to Mnesia.

How

• It could be a store-wide flag to switch the entire store to read-only.
• We could have per-path-pattern flags to provide a more fine-grained solution.
• Another option is to accept a stored procedure to programmatically decide if an update (or any operation in fact) is allowed or not.