Modularized node main function

Main.hs is now Main.hs+Node.hs.

- Node.hs starts a single node, which is parameterized over the
  configuration to use. In the future, this will be useful to have the
  bootstrap server start multiple initial nodes in one program to get
  the network started.

- Main.hs just parses command line arguments to get the configuration,
  and then starts one node.
Remove redundant (= implemented) TODO comments
TODOs edited, cosmetics
Add debug yell message for should-be-dead code
Clarifying comment for partial function