Makes the Supervisor member of Oncilla

This is to removes problems with destruction order. Now the
synchronizer is a member again to control its destruction. It solves
also another problem that may arrived if a user create Oncilla, get a
shared pointer to a node and destructed Oncilla object. Then the
backend would have been unloaded, provoking segfault when using the
shared object. Now the backemnd is also stored statically in a
container, so this problem would only arise if you put a node in a
static variable. But who would do that for god sake ?!