Prestwich/escalator fixes

[prestwich]

Motivation

Pending escalation broken by lack of availability of tx signing on middleware

Solution

Add Middleware::sign_transaction. It errors unless there is a SignerMiddleware in the stack somewhere. The from field has no effect (matching behavior of SignerMiddleware::sign but is added to match the Middleware::sign API for potential future use.

driveby: while testing escalation locally, hit an edge case with tx braodcast failing due to a previous escalation confirming during broadcast. Added a handler for that in the Broadcasting state transition

PR Checklist

• Added Tests
• Added Documentation
• Updated the changelog

[gakonst]

LGTM