If the process we are trying to kill has already exited, don't treat
this as an error. This can happen when we snapshot the process tree
before the process exits but it has exited by the time we get to
trying to kill it.
Fixes#654
This change adds back component output properties. Doing so
requires splitting the RPC interface for creating resources in
half, with an initial RegisterResource which contains all of the
input properties, and a final CompleteResource which optionally
contains any output properties synthesized by the component.
On windows, we have to indirect through a batch file to launch plugins,
which means when we go to close a plugin, we only kill cmd.exe that is
running the batch file and not the underlying node process. This
prevents `pulumi` from exiting cleanly. So on Windows, we also kill any
direct children of the plugin process
Fixes#504
