Reworked config

[zachmu]

This change reworks how the data dir, config file and related initialization values are loaded. The main effect is that it's now possible to run doltgres without a config.yaml file.

The sever config is now resolved in this order

1. --config flag
2. config.yaml file if present
3. Built-in default config values

The data-dir for this config is then overridden as necessary with a similar pattern:

1. --data-dir flag
2. The value in a config.yaml file if, if a file was loaded
3. The env var DOLTGRES_DATA_DIR
4. The default ($home/doltgres/databases)

[Hydrocharged]

LGTM! We should do a small release after this is in