Global Predicates

[justin-atwell]

Currently Scheduled Transactions offer a base level of conditional logic in that if there aren't enough signatures, the transaction does not execute. I propose adding predicates to conditionally execute transactions.

##Motivation

1. This would save users time polling the network but would require the user to subscribe to an HCS topic (I'm not sure on how fees might play into this, but I would certainly pay to avoid having to poll the network and avoid chattiness).
2. This could serve as a way to bridge the gap between traditional "Smart Contracts" and doing things on the user's private network (Like executing a FileDeleteTransaction if the associated Token was updated/deleted).