Move client config to Engine and restore sqitch_db config.

All engines need client, so just put it in the base class. Also, remove the
sqitch_db_uri config, as we just want to allow the name to be set. It should
not have a full path, but just be in the same directory as the main database.
Restore sqitch_db_uri.

More complete, and corresponds to the attribute name, so will be written to
the default config file created by `init`.