[reSIProcate] re-entrant behaviour

[Daniel Pocock]

When an application derives from a class such as InviteSessionHandler, 
and receives callbacks from the stack, are there any limitations or 
recommendations on making calls to the stack while in the callback 
handler?  E.g. is it safe to call all of the methods of 
DialogUsageManager and/or (Client/Server)InviteSession, while processing 
InviteSessionHandler::onConnected(), for instance?