Add Index to SO_Main.BillWhenComplete and SO_Main.CreditNote to improve performance

There is no index on SO_Main.BillWhenComplete Or SO_Main.CreditNote.

Some queries, including those used by our stored procedures such as usp_Jiwa_GetQuantityOpeningBalance which is invoked by usp_JIWA_QuantityAsAt are greatly impacted by this.

Add the following indexes upon upgrade:

CREATE INDEX IX_SO_Main_BillWhenComplete ON SO_Main(BillWhenComplete)
CREATE INDEX IX_SO_Main_CreditNote ON SO_Main(CreditNote)

Make sure the script can deal with the indexes already being there, as these have been deployed to a customers site already.