Trader: support multiple trading configurations at the same time

It will allow to trade on the same instrument, but on different broker accounts. This way it will allow to test different settings.
It will also allow to trade multiple instruments at the same time, on the same broker account or on different ones.

No files, tests, or entry points are named in the issue. Start by locating the Trader implementation and its configuration handling, then determine how multiple broker accounts and instruments should be represented. Done means simultaneous trading configurations work independently and the behavior is covered by appropriate tests.

Written by the indexing model from the issue text.