Split out runtime configuration from the schema

Also use reasonable defaults.
Remove unneeded runner package
Plumb errors through flush()
Don't automatically flush from gumshoe

Leave this to the caller (server). This is a much nicer separation of
concerns and makes some upcoming changes simpler.
Plumb flush errors through to Close() as well
More structure for schema and configuration options

- Split schema from gumshoe runtime configuration from server
  configuration
- Make DiskBacked a first-class notion
Move TOML parsing into package config

Also check that all config fields are defined.
Rename a test file