Replace magic -3 with proper consonstant var

Replaced the magic number -3 that is used in the pullupdate requests with VOID_EVENT_IDENTIFIER
Also created DEFAULT_EVENT_IDENTIFIER to represent the default value used in initialization
The reason I named it "VOID" is because that this pullUpdate event do not require an real event identifier. The identifier could be any int number less than -1.The "VOID "here means "No need for an identifier" (like a void return type of a method).

OF course this name could be rename as "UPDATE_PULLER_EVENT_IDENTIFIER". But then it could only used in the pullUpdate events. For me, it might be better to make it "VOID", then some other event who also do not need a event identifier could also use it. 

Sorry for the indentation, no more next time. Caused by my Eclipse IDE settings.