[Version-unspecific] For streaming swaps, add a comparison of OutTxs to Actions length for Status endpoint outbound_signed, and end display with swap_status if a swap is pending

[Version-unspecific]

Intended to mitigate or close #1686
'BUG: Tx Status Endpoint saying outbound is signed before it is'
for which a streaming swap ObservedTxInVoter doesn't get its later Action/s until the stream completes, and so its status is prematurely set "done".
(The related code is linked here.)

With this MR the Status endpoint will still prematurely indicate completed (but also indicating that a swap is still pending) before the addition of the later Action/s,
but after the later Action/s addition should indicate incomplete until the OutTxs again are as many as the Actions.

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Update:

After further consideration, it seems appropriate to finish the Stages display with swap_status when a swap is pending.

Also, according to this comment streaming swaps could in future use swap queue indices other than 0, in which case a StreamingSwapExists check would be appropriate in checkPending.

Now updated.