ENH: Stream benchmark and treasury data when downloading

Instead of loading entire csv or xml into memory.
BUG: Fix treasury loading.

Make adjustments for using Python built-in ElementTree instead of lxml
based lxml.

lxml was edited out during pulling in of memory friendly loading of
treasury curves, however some of the use of ETree was lxml specific.

Mea culpa.
STY: Add space after comma in order docstring.
STY: Tweak whitespace in comment.
BUG: Fix grouping of events streamed through blotter.

To fix the grouping of events so that (dt, events) ordering
is preserved, the tracking of order states needs to change
in the following way.

Change how order keeps track of dates:
- Change order's dt field to reflect modified date.
- Add a created field.

Change how performance keeps track of orders by:
- Map dt to transactions
- Map dt to orders
- Map order ids to keep track of updated orders.
MAINT: Generate perfomance message only once per bar for minute mode.

Instead of creating a set of perf messages for each event during minute
emission mode, only include the messages on the last event in the bar.

Should cut down on calculations/serialization as well, as work towards
doing more 'end of bar' logic for minute benchmarks.