prserv/db: Threading fixes

prserv/db: Threading fixes

Enabling threading for the PRServer causes a number of issues. Firstly is
the obtuse error:

sqlite3.InterfaceError: Error binding parameter 0 - probably unsupported type

which is due to the class not being derived from object. See:
http://docs.python.org/2/library/sqlite3.html#registering-an-adapter-callable

Secondly, we want to enable multithreadded access to the database so we do this
when we open it. This opens the way up to multithreading the PR server.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>