Since porting all backends to the new `FeedInit` + `MktPair` + `Asset`
style init, we can now just directly pass a `MktPair` instance to the
history endpoint(s) since it's always called *after* the live feed
`.stream_quotes()` ep B)
This has a lot of benefits including allowing brokerd backends to have
more flexible, pre-processed market endpoint meta-data that piker has
already validated; makes handling special cases in much more straight
forward as well such as forex pairs from legacy brokers XD
First pass changes all crypto backends to expect this new input, ib will
come next after handling said special cases..