New functionality for the Mosel language

The type reqqueue can be used to implement multithreaded HTTP servers: it represents a queue of pending HTTP requests to be processed by the server. A queue of this type must be declared as a global shared entity such that each cloned submodel of the master model can access it. A submodel ready to handle a new request has to call httpreqpop in order to warn the queue manager of its availability and then wait for an event. When the server receives a request that has to be processed by one of these submodels it moves this request to the queue using httpreqpushlim or httpreqpush, as a result the request is sent to one of the available submodels that is notified as if the request had been directly received from the network (see Section HTTP server). If no submodel is ready, the request is recorded in the queue until a model becomes available for processing it.