Add unit tests for PortChange notifications. Fix some issue discovered during testing

src/main/java/net/floodlightcontroller/core/IOFSwitch.java

...ain/java/net/floodlightcontroller/core/ImmutablePort.java

...main/java/net/floodlightcontroller/core/OFSwitchBase.java

...va/net/floodlightcontroller/core/internal/Controller.java

.../floodlightcontroller/core/internal/OFChannelHandler.java

.../floodlightcontroller/core/internal/OFSwitchBaseTest.java