Addressing same issue as in #350 where we need to compute position
updates using the *first read* from the ledger **before** we update it
to make sure `Position.lifo_update()` gets called and **not skipped**
because new trades were read as clears entries but haven't actually been
included in update calcs yet.. aka we call `Position.lifo_update()`.
Main change here is to convert `update_ledger()` into a context mngr so
that the ledger write is committed after pps updates using
`pp.update_pps_conf()`..
This is basically a hotfix to #346 as well.
3765c61f2d