Make the Originator a global rather than passing it through arguments

[DavisVaughan]

• Can automatically be applied to "nested" stdin requests that happen within an execute request

• Can be applied to stdin requests that happen from call_frontend_method() when we move that method to the UiComm. Currently, having the originator as an argument makes this move difficult because we can't easily get the originator over to the UiComm (we could potentially ask R_MAIN for it, but that's a little gross).

• We already use a somewhat similar approach when we do IOPub busy/idle through having a "context" for each IOPub message