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
Locating the url of the data is the responsibility of the localize method..