Provide API to adjust schema compatibility after registration

[ivangreene]

Currently the compatibility policy of a schema can only be set during the initial registration of a schema with this fullname. It will be convenient at times to adjust that policy at some point after that registration if needs dictate a different policy.

What are general thoughts on this as a feature? Any considerations for why to only allow setting the policy at registration time? Perhaps this should be a global configuration setting whether to allow updates to policies or not.