Enahancement; Shift `period` to `modifyRailLockup`

[lordshashank]

we should maybe move the period parameter from modifyRailLockup to modifyRailPayment, fixed lockup is completley separate and is not linked to rate and period, while mostly we have used rate * period together to get the payments, it would be better to handle these together only rather than having it with fixed lockup which can become little confusing.

[aarshkshah1992]

@rjan90 Removing this from M2 as it is a nice to have and we can do it after M2 easily.

[DarkLord017]

can i work on this @lordshashank @aarshkshah1992