MAINT: Use pd.Series for benchmarks and algorithm returns in risk.

Instead of lists, use pd.Series, so that memory is preallocated.
ENH: Stream benchmark returns as events.

Instead of creating a list of benchmarks in the risk module,
stream benchmarks through the system as events, starting from the
algorithm generator.

Works towards more easily setting arbritrary pricing data as
a a benchmark, as well as working towards live minutely benchmarks.