add a GetCloseoutPrice method to the caching

[jeremyletang]

The caching layer was caching the right function, although the orderbook is not aware of the cached value when calling GetCloseoutPrice, this now add GetCloseoutPrice into the caching latyer.

This was indeed pretty bad, I thought the GetCloseoutPrice function was called in much more places, and it would required more changes from the risk engine.

Luckily it does not, so now we just call the appropriate function when needed instead of letting the matching engine figure out what to do.

[ashleyvega]

@jeremyletang I see a new func (b *CachedOrderBook) GetCloseoutPrice(...) {...} function, but I don't how/where it's called.

[EVODelavega]

@ashleyvega it was called by the risk engine, but no matter: we've got a different fix