fix(PaymentSession): find minSendAmount independent of rate

[sidvishnoi]

Context

Part of #976
Extracted from #1046

Changes proposed in this pull request

• When a new link tag is added, we want to keep using older minSendAmount, even if the adjustAmount may change due to splits. adjustAmount resulting rate is >= minSendAmount, so it makes sense to find minSendAmount first, and not find it ever again.

Known existing issues (discovered while working this out):

• Currency fluctuations can change this minSendAmount in future. In that case, we'd need to probe/findMinSendAmount again.
• Exponential probing is likely not the best approach. (Raw thoughts follow) I wonder if we can use currency conversion and then binary search or two-way search some values (specially relevant when sending from e.g. MXN to USD: with exp probing, we go 1, 2, 4, 8... with some two way search, we can go like: 21 (ceil exchange rate), 22, 19)
• Can we only pay in multiples of minSendAmount? Need to check how OP/Rafiki behaves in this scenario. For example, when sending ZAR to USD (20 units vs 1), minSendAmount can be 20. Rate of pay can say we've to pay 30, but we can pay either 20 or 40 (1 or 2 units only). OP allows this, but is this optimum distribution?

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Latest commit	2c57f4f
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[sidvishnoi]

This ensures we call _findMinSendAmount() only once per PaymentSession instance.

[DarianM]

test-e2e

[sidvishnoi]

Aside: I really need to figure out page.evaluate: Execution context was destroyed errors in E2E!