BasicConfig support for legacy values:

* A legacy value is a config section with a single-line.
* These values may be read from the BasicConfig interface so
  the deprecated Config class does not need to be exposed to
  clients.
* Made Config class more testable.

doc/rippled-example.cfg

@@ -698,7 +698,8 @@
 #   There are 4 book-keeping SQLite database that the server creates and
 #   maintains. If you omit this configuration setting, it will default to
 #   creating a directory called "db" located in the same place as your
-#   rippled.cfg file.
+#   rippled.cfg file. Partial pathnames will be considered relative to
+#   the location of the rippled executable.
 #
 #
 #