Since `open_trade_ledger()` now requires a sort we pass in a combo of
the std `pendulum.parse()` for API records and a custom flex parser for
flex entries pulled offline.
Add special handling for `MktPair.src` such that when it's a fiat (like
it should always be for most legacy assets) we try to get the fqme
without that `.src` token (i.e. not mnqusd) to avoid breaking
roundtripping of live feed requests (due to new symbology) as well as
the current tsdb table key set..
Do a wholesale renaming of fqsn -> fqme in most of the rest of the
backend modules.
Tyler Goodlet