MAJOR: servers: propagate server status changes asynchronously.

MAJOR: servers: propagate server status changes asynchronously.

In order to prepare multi-thread development, code was re-worked
to propagate changes asynchronoulsy.

Servers with pending status changes are registered in a list
and this one is processed and emptied only once 'run poll' loop.

Operational status changes are performed before administrative
status changes.

In a case of multiple operational status change or admin status
change in the same 'run poll' loop iteration, those changes are
merged to reach only the targeted status.