bitbake: prserv/db: Threading fixes

bitbake: 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.

(Bitbake rev: 5709efc2ff1e36529bd28f49cd093ccfa7abff7f)

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