Some tidyups to address

[zzlyn]

• go through the case where a node didn't receive a matching pre-prepare message
• need to convert most of the data structures where messages are being pushed onto an array into single elements. so e.g. acceptedPrePrepares[view][sequenceNumber] should return a single object, not an array of objects
• possibly don't need to track messages in all views at each server
• show proper fields for each message and node type in the message and node modals

[zzlyn]

Original Issue: zzlyn/ConsensusVisualization#76