DRILL-7620: Fix plugin mutability issues
A recent commit made the plugin registry more strict about
the rule that, once a plugin is registered, it must be
immutable. A flaw enforcing that rule in the UI put the
registry in an inconsistent state.
Also
* Registry-specific errors
* Push more operations from UI layer into registry
* Clean up semantics of "resolve" for plugins
* Add more unit tests
* Better handling of "bad" plugins
* Force plugin names to lower case
* Fix comparison bugs in some format plugins
