avoid null route, ref, and status in new exchanges

[chadwhitacre]

See #3810 for why cross-linking transactions is a good idea. We were doing this for Balanced by the end, but I don't think we're doing this for any payment networks now.

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[rohitpaulk]

We had the 'reverse' of this in Balanced, iirc. There was a meta column named 'exchange_id' on every transaction in Balanced.

[chadwhitacre]

@rohitpaulk Yes! We preferred that on #3807:

gratipay.com/backfill-exchanges.py

Lines 124 to 129 in e236680

	# First strategy: use the one in the transaction!
	exchange_id = transaction['meta'].get('exchange_id')
	if exchange_id:
	counts.exchange_in_transaction += 1
	log("exchange_id in transaction", 26)
	return exchange_id

[chadwhitacre]

Really we should refactor payment routes along the lines of accounts elsewhere, to have a more consistent interface across all of the routes we support, to include method and transaction linking, and status changes via postbacks.

[chadwhitacre]

We haven't had a NULL route (at least) since #3810. See #3975 (comment).

[chadwhitacre]

Folding this into #4442.