split IDisplayableServer from IServer, add sb.get_stub_server()

split IDisplayableServer from IServer, add sb.get_stub_server()

IDisplayableServer includes just enough functionality to call
.get_name() and friends, which is all that the UploadResults really
need. IServer is a superset that includes actual share-manipulation
methods. StubServer instances provide only IDisplayableServer, while
actual NativeStorageServer instances provide the full IServer interface.

When the Helper sends a serverid (so we know what to call the server but
nothing else about it, and have no corresponding NativeStorageServer
object to reference), but we want to store an IDisplayableServer in the
UploadResults, we create a synthetic StubServer "server" and store that
instead.