Avoid a 'used once' warning
Cached/calculated properties (memoized) now have a method to clear a previous calculation

It's named __invalidate_<accessor>__
Add a function to always return an object

Works whether it's passed a regular object, singleton object or
singleton class name
Refactor to harmonize data source methods around handles

Instead of get_default_dbh or init_dbh, the public methods are now named
get_default_handle, disconnect_default_handle, has_default_handle

Datasources should implement create_default_handle and disconnect

Send signals before and after datasource connect and disconnect:
    precreate_handle create_handle
Implement disconnect() for UR::DataSource::FileMux
Refactor to add a method to get the current file's position
whitespace
Speed up the test by not sleeping before exiting
Don't double-call init_created_handle()

UR::DataSource::create_default_handle_wrapper() already calls it
Add a test for the new signals sent by UR::DataSource