ENH: filter current orders via the time in force models

Current orders in the blotter were determined by filtering the orders
that have a dt attribute <= current trade event dt. Now they are
filtered using the orders time in force model.

The filter only takes orders where the trade event falls within the
valid date range for the order. The old criteria is also kept, but is
probably redundant.
ENH: separated first/last dt checks for order filtering

Renamed the variable names to imply that the first and last dates are
also valid dates.

Separated the valid dt check to return a tuple. Currently the tuple
returns the following.

(first valid date <= current_dt, last valid dt >= current_dt)