MAINT: Simplify current date logic in tradesimulation loop.

Now that the tradesimulation loop has changed to use benchmarks
as a 'clock', the logic for setting the current time can be grouped
together at the beginning of each iteration instead of the date
and snapshot grouping.

Also, can remove the snapshot_dt and use simulation_dt instead
of having two variables that were keeping track of the same value.

Also, it is no longer needed to peek into the data to get the first
simulation_dt now that simulation_dt is set at the beginning of each
loop iteration.
MAINT: Remove logging processer from transaction stub.

Now that the tradesimulation transform calls the blotter/slippage
within, instead of being wrapped in a slippage generator, the logging
for slippage can rely on the processor set within the tradesimulation
loop, instead of maintaining its own.