Remove bottleneck caused by unnecessary check of the position index.

Instead of checking the positions indexes every time either
`_position_amounts` or `_position_last_sale_prices` is updated, check
and grow the individual Series on each update.

This gain with this patch is by reducing the following bottlenecks:
- Checking both vectors when only one is updated.
- Using try/except to trigger the growth, instead of incurring the cost
of checking the Index contains on every update.

In testing this change results in about a 33% speedup of the
`update_last_sale` algorithm when run with a buy and hold algorithm with
160 equities, resulting in a 20% speedup overall.

## Further Context

The eventual goal here is to collapse the two position Series into a DataFrame, as prototyped here https://github.com/quantopian/zipline/tree/vectorize-positions, but the gain from this patch had a big enough gain to parts changed that it is worth pulling in before doing more volatile changes.

That would be possible, but I don't believe have any invocations that in practice would cause that out of sync issue.

In zipline the function is called as:

```
self.update_position(event.sid, last_sale_price=event.price,
                                 last_sale_date=event.dt)
```

Which is guarded by a check `event.sid not in self.positions` which would rule out the 'no amount' case.

In our internal code there are two calls which do not include both values:

```
perf_period.update_position(sid, amount=broker_amount)
```

and 
```
perf_period.update_position(sid, amount=0, cost_basis=0.0)
```

However those iterate over existing positions in the perf_period which mean that they have both amount and last_sale_price in the vectors already.

It is theoretical that down the road usage of this function could change, and argument could be made that we should defensive about that potential drift, but not sure if it is worth it with the other changes we have discussed.
(And that may be a reason to wait for the other changes since they would obviate this case.)