Despite a `.bs_mktid` ideally being a bijection with `MktPair.fqme`
values, apparently some backends (cough IB) will switch the .<venue>`
part in txn records resulting in multiple account-conf-file sections for
the same dst asset. Obviously that means we can't allocate new
`Position` entries keyed by that `bs_mktid`, instead be sure to **update
them instead**!
Deats,
- add case logic to avoid pp overwrites using a `pp_objs.get()` check.
- warn on duplicated pos entries whenever the current account-file
entry's `mkt` doesn't match the pre-existing position's.
- mk `Position.add_clear()` return a `bool` indicating if the record was
newly added, warn when it was already existing/added prior.
Also,
- drop the already deprecated `open_pps()`, also from sub-pkg exports.
- draft TODO for `Position.summary()` idea as a replacement for
`BrokerdPosition`-msgs.
b0f273f091