This refactoring brings in device Cache running in its own
thread and scanning FPGA devices once every 5 secs. Then no timers
are used inside ListAndWatch() method of device managers and
no need to run scanning periodically in every device manager's
thread.
Cache generates update events and the plugin creates, updates or
deletes device managers on the fly upon receiving the events.
Introducing new modes of operations is a matter of adding a single
function converting and filtering the content of Cache.
