Calling Fdb.Stop() throws PlatformNotSupportedException

[KrzysFR]

The network thread is executed from a dedicated managed thread that is created at the start, and calls into fdb_run_network.

To stop it, the c# code calls fdb_stop_network and, hopefully, the fdb_run_network should eventually return and give back control to the CLR.

The current code does wait for up to 5 seconds, before calling thread.Abort() and then thread.Join(30sec).

The issue is that Thread.Abort() is deprecated and not supported in .NET anymore, and calling Abort() will throw a PlatformNotSupportedException.

Calling abort on the managed thread would not really do anything anyway because it cannot pass through this event to the native, so I don't think it helps much, so it could probably be removed altogether.