Improve section headers and add state descriptions

Summary:
Improve comment headers dividing sections of the RAFT server source and
add descriptions for the six `gen_statem` states defined by the RAFT
server implementation.
Reorder methods to match export order.

Differential Revision: D59696033

fbshipit-source-id: 0d8eb48829de1aa933e038ab2969f36979f484cf