When running in the region mode we don't need to know AFU IDs
thus don't read them while in the mode.
It's important not to try to read them because in the region mode
AFUs are supposed to be reprogrammed in the fly anyway and the
afu_id files may become busy.
In the `af` mode the plugin announces AFUs and tells kubelet
to pass only AFU ports to containers.
In the `region` mode the plugin announces region interfaces and tells
kubelet to pass only AFU ports to containers.
In the `regiondevel` mode the plugin announces region interfaces and
tells kubelet to pass AFU ports and FME devices to containers, so the
conteainers have full access to the regions.
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.
