This PR refactors most of the codebase to use the new config patterns implemented in #149. Now the core service keeps a copy of the KolideConfig struct, and service methods can reference the configuration in that struct when they need it. The most significant refactoring is in the sessions code, separating the business logic from the storage layer.
This provides a new pattern for defining and loading configuration for the Kolide server. With this PR, configuration is stored in the `config.KolideConfig` struct, and loaded through `config.Manager`. Refer to the patterns in `config/config.go` and `cli/prepare.go` to see how configuration is defined and used.
A follow-up PR will work on removing further references to `viper` throughout the codebase, instead preferring to access configuration through the strongly typed `KolideConfig`.
