EDBI operation improvements

* introduced API methods for
    * retrieving all Index instances
    * removing an Index and related schema objects
* improved `Operation` class hierarchy in edbi jdbc implementation
* implemented drop methods for schema objects

I was looking just at the changes and haven't seen the methods where the operation subclasses are handled. however, i still don't understand why the fallback method doesn't throw an exception (e.g. UnsupportedOpExcp,  IllegalArgExcp, or IllegalStateExcp)