[Matching Engine]: Update for reserve stocks

[nimishagarwal76]

Previously, in case of PerformOrderFillTransaction, askTransaction would have negative stockTradeQty which would get sent to the datastream. Now, it's 0 as user doesn't actually have to lose stocks anymore.
HACK:
Re-setting the object just before sending back the tranasction ensures that the rest of the matching engine / order book code functions without requiring changes.

Need to fix this.