Allow separate stacks registration through services operation

Until now, stacks could not be delta-loaded. All the stacks might be
loaded in a unique single xml file (or xml representation through
rest api). This was due to counterscope initialization restriction:
once a counter was registered (due to event received), you could not
initialize more counter scopes.

We removed this restriction (now commented on initializeCountersScope).
New scopes() method to know current registered ones in order to put
the correct start point at services load procedure.

The services "node" nodes always could be loaded separately, so, no
impact on functional behaviour for this.

Fixed: https://redmine.teslayout.com/issues/48