Unified configuration file using conf_d module

This change adds support for a conf.d directory - configured only via the `--confd-path` flag - which allows beaver to read configuration from multiple files.

Please note that the primary `beaver` stanza MUST be located in the file specified by the `--configfile` argument. Any other such `beaver` stanzas will be ignored.

This change also unifies the `BeaverConfig` and `FileConfig` classes, and simplifies the api for retrieving global vs file-specific data.

Please note that this commit BREAKS custom transport classes, as the interface for creating a transport class has changed. If you are referencing a `file_config.get(field, filename)` anywhere, please omit this and refer to `beaver_config.get_field(field, filename)`.

Closes #107
Respect stat_interval file configuration in stable worker