Extensive refactor and clean up

[jhalter]

• Replace use of hardcoded data types in server struct with more flexible interfaces that can be swapped out to allow for different backend data source (e.g. accounts in SQL DB instead of yaml files)
• Replace use of server mutex with more granular mutex
• Misc code re-org and tidy to prepare to split out hotline lib from server implementation
• Initial implementation of SIGHUP handling to reload config
• Fix #52