Allow the Open/CreateDB funcs to take any params.

The specific parameters required by a backend is better left up to the
backend itself.  For example memdb has no need for a database path, while
ldb does.  This commit modifies the OpenDB and CreateDB functions to take
a arbitrary arguments which are passed along to the driver.  The driver is
expected to verify the correct type and number of arguments and error
accordingly.

The existing backends have been updated accordingly.