Protocol: support user disposing handles that are sent

[Tom Deseyn]

Consider:

SafeHandle safeHandle = ...; // 1

messageWriter.WriteHandle(safeHandle);

connection.TrySendMessage(message); // 3

The library ensures that the safeHandle passed in // 3 gets disposed.
When an exception occurs between //1 and //2 the safeHandle won't get disposed.

We should allow the user to dispose the SafeHandle:

using SafeHandle safeHandle = ...; // 1

This isn't supported currently because // 3 can return before the message was effectively sent, in that case we dispose the SafeHandle before sending it.