PERF: Remove deepcopy from EventWindow ticks.

The deepcopy of events into the EventWindow's ticks was causing
a significant increase in memory consumption, e.g. an algorithm with
almost 200 sids and 14 vwaps removing the deepcopy reduces the amount
of memory consumed by about 40%.

The downside is that if an event's properties are changed, which is
not advised, later on, then the signal derived from vwap etc.
may be changed.