Refactor configuration

[bilfeldt]

Flatten and refactor the configuration:

• filtering.always is renamed to filter_all
• filtering.2xx is renamed to filter_2xx
• filtering.3xx is renamed to filter_3xx
• filtering.4xx is renamed to filter_4xx
• filtering.5xx is renamed to filter_5xx
• filtering.slow is renamed to filter_slow
• log_to_channel.enabled has been removed, instead logging to channel is enabled when a channel is provided
• log_to_channel.channel is renamed to channel
• log_to_disk.enabled has been removed, instead logging to disk is enabled when a disk is provided
• log_to_disk.disk is renamed to disk
• log_to_disk.separate is renamed to disk_separate_files
• log_to_disk.timestamp is renamed to prefix_timestamp
• log_to_disk.filename is renamed to filename

The following environment variables have been renamed:

• HTTP_CLIENT_LOGGER_FILTERING_ALWAYS is renamed to HTTP_CLIENT_LOGGER_FILTER_ALL
• HTTP_CLIENT_LOGGER_FILTERING_2XX is renamed to HTTP_CLIENT_LOGGER_FILTER_2XX
• HTTP_CLIENT_LOGGER_FILTERING_3XX is renamed to HTTP_CLIENT_LOGGER_FILTER_3XX
• HTTP_CLIENT_LOGGER_FILTERING_4XX is renamed to HTTP_CLIENT_LOGGER_FILTER_4XX
• HTTP_CLIENT_LOGGER_FILTERING_5XX is renamed to HTTP_CLIENT_LOGGER_FILTER_5XX
• HTTP_CLIENT_LOGGER_FILTERING_SLOW is renamed to HTTP_CLIENT_LOGGER_FILTER_SLOW
• HTTP_CLIENT_LOGGER_CHANNEL_LOG_ENABLED removed in favor of HTTP_CLIENT_LOGGER_CHANNEL
• HTTP_CLIENT_LOGGER_DISK_LOG_ENABLED removed in favor of HTTP_CLIENT_LOGGER_DISK