Refactor server

Refactor server

- server can be used as a Go library
- unexposed type and values that aren't consumed outside package
- golint fixes
- switch logging to logrus
- add more logging messages
- improve usage as a library: no more `panic` or `os.Exit`, return `error` instead
- evalute/raise nearly all possible `error` values
- use interface to allow library consumer to use custom peer configuration backend