[Transactions]: Add constraint for preventing short selling after some extent

[abesheknarayan]

This year , we tried to add a constraint where we check in each transaction if the user has enough cash to retrieve the stocks after short selling, but in the main event as the no of transactions were high, we faced some sql lock timed out issues during first day.
Have to think about better way of doing this and also we need to write stress tests to check with lot of concurrent transactions.