Remove heavy operation in the context of OSGi framework

Remove heavy operation in the context of OSGi framework

The routine modeChangeNotify can be called in the context of the OSGi
framework via the chain:

<snip>
at org.opendaylight.controller.forwardingrulesmanager.internal.ForwardingRulesManager.modeChangeNotify(ForwardingRulesManager.java:2137)
at org.opendaylight.controller.switchmanager.internal.SwitchManager.switchManagerAwareNotify(SwitchManager.java:1797)
at org.opendaylight.controller.switchmanager.internal.SwitchManager.setSwitchManagerAware(SwitchManager.java:1677)
</snip>

now given the modeChangeNotify trigger an interaction with the network
element, it's better to keep that off a different thread to avoid to
stuck the OSGi framework for to long.

Change-Id: I41ba96372816cd0a5b2b488bbfc64d10ba2adb4a
Signed-off-by: Giovanni Meo <gmeo@cisco.com>