Changed default database creation behavior

[Hydrocharged]

Previously, when running doltgres, it would look at the current directory to determine if it was a valid DoltgreSQL directory. If it wasn't, then it would attempt to create a doltgres database within the directory. This behavior was chosen since "it just works", which is a selling point of the product. This was causing issues though, as users would run the tool from within the same directory that they downloaded it in. This caused an error message to appear stating that the file was conflicting with the attempt to create the database, however users were still confused by the behavior even with the error message.

This default behavior has been changed. We now use the ~/doltgres/databases directory as the default directory. It no longer matters where you run doltgres from, it will always point to that directory. This directory can be changed by changing the environment variable DOLTGRES_DATA_DIR to a different directory.

In addition, there's another environment variable named DOLTGRES_DATA_DIR_CWD. When this is set to true, then it causes DoltgreSQL to operate with the previous behavior, in that it will use the current directory rather than the global data directory.

[zachmu]

LGTM, main comments:

1. One env var is better than two, just allow . and everything works
2. --data-dir needs to take precedent if it's provided, don't see any tests of that