Reorganize & clean exchange-rate logic

[swansontec]

Pull request #5423 contained a bunch of logic errors, so clean everything up in a way that makes it less confusing and less buggy.

• Break up long lines.
• Fix misleading comments.
• Give variables better names.
• Don't copy data needlessly.
• Stomp old cache entries instead of mutating them.
• Only add the account fiat pair once, instead of in two separate places.
• Expire cache entries once a load-time, instead of doing this in three separate places.
• Do not store reverse rates in the cache, but derive them at the very end.
• Move getYesterdayDateRoundDownHour into this file, since it's only used here.

CHANGELOG

Does this branch warrant an entry to the CHANGELOG?

• Yes
• No

Dependencies

none

Requirements

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• Tested on iOS device
• Tested on Android device
• Tested on small-screen device (iPod Touch)
• Tested on large-screen device (tablet)

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