Make handles immutable

- incoming handle IDs are always ignored in insert_handle, since it
should be a new handle
- to update a handle, the user must provide the same id, type, url, and
hid, and they must match the DB record (this might need rethinking,
another option is to just ignore those fields and only require the hid)
- the id, type, url and hid cannot be altered on a persist_handle call
- throw an error if the persist operation fails due to non-matching
record
- made some changes in initialize_handle to properly set the url and
type, given they're implicitly determined in the method
What confused me is that the initialize_handle method ignores the url provided by localize_handle and always uses $default_url. So with the prior behavior, a user could pass a handle with an alternate url to initialize_handle, get the handle back, and it wouldn't work because the url that was initialized is different than the url in the provided handle.