Consider letting the operator change the "to" address of rails

[Stebalien]

This might be helpful in a few situations:

1. Repair: need to start paying some other party.
2. Async deal flows: the operator may want to lock funds ahead of time, but doesn't yet know to whom those funds are to be paid.

I want to mull over this a bit, but I'm filing it as an issue so we don't forget it.

[Stebalien]

@aarshkshah1992 this is important if we want to support buying storage from "pools".

[aarshkshah1992]

Allowing this also opens up a whole bunch of security questions so we should punt on this unless we absolutely need it.