New rules for `Close` messages

[KendallWeihe]

Intentionally creating a bit of a duplicate from #242 with the hope of focusing aim and consolidating consensus requirements

1. Close messages are valid after any message type
2. Close message introduces a new optional property success of type boolean

The logic of Close.success is as such:

if Close.success == null then that means Close was sent prior-to the Order execution
else if Close.success == true then Close was sent after a successful Order execution
else if Close.success == false then Close was sent after a failed Order execution

Close after Order

Close messages may be sent by Alice to the PFI during an actively executing order (AKA a state after, but not necessarily immediately after, an Order but prior to a Close where the Close.success is true), but that doesn't necessarily guarantee a successful canceled execution.

In the event Alice sends a Close message during an actively executing order:

if Close.success == true then Alice failed to cancel the executing order
else if Close.success == false then Alice successfully cancelled the executing order

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1. We need to update the relevant spec docs here in this repo
2. We need to trickle through the changes through to various implementations

[KendallWeihe]

@phoebe-lew are we actively labeling these for 1.0? This one doesn't have a label.

Edit: nevermind I see it now, it's a Deadline

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Two parts to this:

1. Ensure the success property is within the Close message
2. Notify DRI's of the necessary changes for the given implementation (Note: Order messages are valid after any message, see description, so ensure that the various "valid next" calls enable that support)