Improvements to port allocation support

Applications like Storm require dynamic port allocation. However, the allocated port may not get immediately used - e.g. till a Topology is deployed. In general, there is always a small window between Slider allocating a port and the application actually using it.

The ideal solution is some form of port reservation system that is OS backed. But in absence of such a capability Slider should ensure that allocated port is reserved for the application instance till the application is active independent of whether the application actually uses it.