ThaliCore: implement max VirtualSocket limit

[enricogior]

The max number of overall connections in the native layer is limited by the number of concurrent virtual sockets: each virtual socket adds an entry to RunLoop and the max number of entries per process is 64, that is enforced by the OS.
Adding more than 64 entries doesn't trigger an error in RunLoop, simply the entries after the 64th will not be scheduled until other entries have been removed.

Given one entry in RunLoop is alrady taken for another workitem, it would be safe to limit the max virtual sockets to 62.