Modify Catalog.get_store() to more efficiently find store of a given name

A GeoServer workspace can't have two stores of the same name, so requesting both the datastore_url and coveragestore_url each time get_store() is called, doesn't seem to be necessary.  This change just checks datastores first, then coveragestores for stores matching the 'name' parameter.

Since get_store ends up being called often in other methods, this change can reduce quite a bit the number of REST API calls for a given operation.