DB: Add trigger on split

[victorbrun]

When new split is introduced the holdings of that instrument needs to be updated to counter the effect of adjusting prices. The trigger ought to be implemented as follows:
New split of instrument that is in holding table -> new transaction is created for each old transaction before split date. These transactions are on the same date as respective original transaction but at price 0 and quantity n*split_factor, n is quantity of original split.

Furthermore, transaction to holding trigger needs to be changed so the above, retroactive transactions, effect the holdings even though if they are closed.