In CalciteConnectionConfigImpl, add "isSet" and "unset" methods

Currently the "set" method in CalciteConnectionConfigImpl has no effect if the property is already set, and most people would find that behavior surprising.

This change adds "isSet" and "unset" methods, and restores "set" to the obvious behavior. If you want to set conditionally, you can check "isSet" first.

Using isSet, Planner.connConfig() is now able to merge parser config and connection config in the way that you would expect. A property value in parser config will override connection config only if the corresponding property in connection config is not set.

The code in CalciteConnectionConfigImpl.set previously called new Properties(properties) (whose behavior is a little confusing, because it sets defaults) and now calls Properties.clone() (which is more straightforward).