Since it's depended on by `.data` stuff as well as pretty much
everything else, makes more sense to expose it as a top level module
(and maybe eventually as a subpkg as we add to it).
It's finally a decent little design / interface and definitely can be
used in other backends like `kraken` which rolled something lower level
but more or less the same without a wrapper class.
Connecting to a `brokerd` daemon's trading dialog via a helper `@acm`
func is handy so that arbitrary trading middleware clients **and** the
ems can setup a trading dialog and, at the least, query existing
position state; this is in fact our immediate need when simply querying
for an account's position status in the `.accounting.cli.ledger` cli.
It's now exposed (for now) as `.clearing._ems.open_brokerd_dialog()` and
is called by the `Router.maybe_open_brokerd_dialog()` for every new
relay allocation or paper-account engine instance.
Tyler Goodlet