Finish core Raft consensus implementation

[heidihoward]

The following components are still necessary:

• client request cache
• state machine failure
• dispatch append entries immediately after a client request or a negative append entries response is received, expose existing implementation as a "batch append entries" option in advanced settings
• testing!

The following would be nice to add in the future:

• dynamic membership
• log compaction
• no-op at start of leadership?
• add separate election duration timer
• add client request acknowledgement and timer
• handling read-only commands