make prepare phase smarter

[bdeggleston]

• If the prepare responses are all preaccepted instances with the dependency match flag set, prepare should send a commit
• If the prepare responses are all preaccepted instances with the same ballot, the dependencies should be merged and an accept message should be sent.
• If the prepare responses are all accepted instances with the same ballot, seq, and deps, a commit message should be sent
• If any response has a committed or executed messages, instance should be committed locally