Allow bypassing `readonly` via configuration parameter

[CodesInChaos]

In my experience we sometimes end up with incorrect data in readonly fields, through bugs or user error. This data then needs to be cleaned up via some kind of one-time maintenance script. Currently there is no way to do that, short of making the field writable, modifying it, and then making it readonly again.

So I'm proposing a way to bypass readonly. I think a per connection override allow_modifying_readonly, similar to apply_access_policies would be a sensible place.