Revert config loading refactors

[mraerino]

Reverts these PRs:

• provide default config for logging (provide default config for logging #183)
• Fix usage of struct tags for config (Fix usage of struct tags for config #203) - basicly a follow-up to the first one which didn't work

I was getting this error at proxy startup when updating this lib:

Failed to parse configuration: failed to map data: 11 error(s) decoding:\n\n* 'launch_darkly.request_timeout' expected type 'time.Duration', got unconvertible type 'string'\n* 'log_conf.quote_empty_fields' expected type 'bool', got unconvertible type 'string'\n* 'origin_limits.backoff_ratio' expected type 'float64', got unconvertible type 'string'\n* 'origin_limits.enabled' expected type 'bool', got unconvertible type 'string'\n* 'origin_limits.increment' expected type 'int', got unconvertible type 'string'\n* 'origin_limits.minimum' expected type 'int', got unconvertible type 'string'\n* 'origin_limits.slowstart' expected type 'int', got unconvertible type 'string'\n* 'origin_limits.timeout' expected type 'time.Duration', got unconvertible type 'string'\n* 'timeouts.function.response_header' expected type 'time.Duration', got unconvertible type 'string'\n* 'timeouts.origin.response_header' expected type 'time.Duration', got unconvertible type 'string'\n* 'timeouts.proxy.response_header' expected type 'time.Duration', got unconvertible type 'string'

we might be able to avoid this when switching to json or yaml struct tags, but this would mean a lot of changes in different services and i don't think the effort is justified right now.

cc @rybit