MAINT/PERF: Make the portfolio property call `updated_portfolio`.

Make the portfolio property on TradingAlgorithm call `updated_portfolio`
internally.  This prevents needless recomputation of the portfolio between
calls to `handle_data`, and also prevents issues where the portfolio object
could be unexpectedly modified in place in the body of a `handle_date` call.

Also adds some optimizations to `PerformancePeriod.get_positions` to 
avoid needless insertions/deletions from the `Positions` object that's supplied
on the algo-facing portfolio.