clean up public api

[willscott]

there are a bunch of internal configuration that have ended up exposed as public parts of the config.

for instance:
StrnEnvKey
DefaultLoggingInterval
ErrBackendFailed

etc.

These are used internally to this package, but are not something that a consumer of caboose needs to care about.

We should clean up the exported interface and document it so that another user can pick it up easily