[cf-core] Validation Middleware

[LayneHaber]

Description
Currently, there is no way to inject validation into the node. This is a problem, particularly for uninstall, when users may be able to prematurely uninstall apps. Client and node should be able to inject middleware to be executed prior to state transitions within the lower level frameworks.

Solution
Add an injectMiddleware function to cf-core that will be run prior to the state transitions in each protocol in response to an OP_VALIDATE call.

If the callback passed in errors, the protocol should fail before updating the store.