Implement skipping to DECIDE phase after receiving single DECIDE message

[anorth]

A DECIDE message carries justification of a strong quorum of COMMITs, and so is enough to convince a node to decide the same value immediately, and send its own DECIDE message. Implement this shortcut.

This is a specific case of a more general ability to use the justification in place of receiving the messages that it is evidence of (e.g. a COMMIT with a strong quorum of PREPAREs).